Answer is A.

It is because they are actually fossils. They are made of dead and decay matter buried under the earth for many years. The dead and decaying matter under great amount of pressure and temperatures turn into fossils and they both are used as fuels so it is called fossil fuels.
The fossil fuels we use today are actually formed from plants and animals that existed hundreds of millions of years ago. The reason there is a lot of concern surrounding the expenditure of these resources is because most of what we use today from fossil fuels, such as oil and coal are being depleted much faster than they can be formed, so there is a limited amount available.
Hence, the statement is true.

Fossil fuels are formed when plants and marine species are buried below ground and are subjected to immense pressure and temperature for thousands of years. Examples include coal, petroleum and natural gas.

A solar cell gathers sunlight. It is the light energy which is converted into electrical energy.

Solar panel refers either to a photovoltaic module, a solar hot water panel, or to a set of solar photovoltaic (PV) modules electrically connected and mounted on a supporting structure. A PV module is a packaged, connected assembly of solar cells. Solar panels can be used as a component of a larger photovoltaic system to generate and supply electricity in commercial and residential applications.

In the box type solar cooker, the heat from the sunlight is used to cook the food.

Solar water heater works on solar radiation falling on it. It requires solar light for generating enough heat to heat water. During cloudy days, Sun is obstructed by clouds, and the light incident of the solar panels will be inefficient to make the heater work.

Solar heating device first converts heat energy of Sun to heat up a substance, which is then converted to electrical energy.

Solar panel uses multiple solar cells to increase electricity produced by the system.

Solar panel uses multiple cells to add up electricity output from the system.

Solar cell converts light energy of sun into electrical energy.

Solar cells have access only to the solar energy and chemical energy (battery embedded in satellite system). However, chemical energy is limited. Hence, solar energy is the standard source of energy used in satellites.

Group of solar cells is called solar panel.

Solar cell produces pure electricity with no waste products and is environment friendly. 

When light energy is incident on Si, it is converted to electricity by the principle of photovoltaic effect. GaAs, InP are other materials with similar property. 

The cost of solar cells is reducing exponentially.

Solar energy production on a large scale is difficult on a large scale because:

Black surfaces are good absorbers and good radiators of heat and hence painting the surface black traps more heat and improves the efficiency of solar cooker.

The reflector on the top of the solar cooker is used to focus the solar energy over the glass surface. The mirror reflects the heat energy in a confined area and increases the net input energy to the solar cooker.

Solar cells are fitted on a scientific calculator.

Production of solar cells is increasing exponentially every year.

Solar cell works on the principle of photovoltaic effect according to which free electrons are produced when light of certain wavelength is incident on semiconductors.

Rotation of Earth about the Sun causes the position of Sun to change during the day. Hence, the mirror on the top of the solar cooker needs adjusting during the day to focus the rays in a small area.

Solar cooker is a device used to collect the solar energy in the form of heat.

To improve the efficiency of a solar cooker, the following steps need to be done:

Solar cooker is a device that uses energy from the Sun directly to cook food or warm fluids.

Concave or plain mirror are used on the top cover of the cooker. Light get reflected from these mirrors, these mirrors concentrates the light onto a point or a region where the food is kept to be cooked.

Concave or plain mirror is used in the solar cooker in order to concentrate the sunlight at one spot.

Interior of the solar cooker is painted black inorder to absorb maximum of the Sun's light falling on the device.

Solar panel is a very useful device. Some of its major advantages are:

Solar devices are dependent on the sunlight for the generation of electricity. For the solar device to work efficiently, sunlight with good intensity should be available. Sunlight is weak during cloudy and rainy days. 

Solar panels are used to convert light (from solar energy) into electricity.

Solar panels can be used in calculators and battery operated toys, in satellites and Mars probes to provide them with energy to function in outer space by using the Sun's rays, can be used in traffic signals etc.

Solar panels are dependent on the sunlight for the generation of electricity. For the solar panels to work efficiently, sunlight with good intensity should be available to fall on the panels. Sunlight is weak during cloudy and rainy days. Hence, solar panels cannot produce energy throughout the year.

Solar constant is defined as the mean amount of the solar energy received by the Earth's atmosphere, per unit of the area, during the time when the Earth is at the mean distance from the Sun. Solar constant equals to 1388 watts per square meter.

Solar cells are devices that converts Sun's radiation energy into electricity. Solar cells contains a semiconductor. Silicon is a good semiconductor that can be used for the production of solar cells.

A solar water heater cannot be used to get hot water on a cloudy day as there is no Sun which is necessary for the working of solar water heater.

In parabolic solar cooker, the container of cooking is placed at the focal point of mirror so that all rays that are falling on solar cooker can converge to one point and produce large amount of heat energy.

The device used in traffic signal lights is Solar traffic light. are signalling devices powered by solar panels positioned at road intersections, pedestrian crossings and other locations to control the flows of traffic. 

Water always flow due to pressure difference generated by heat in solar cooker.

1. Clean the glass cover regularly .
   2.Prevent nay shade of the collector.
   3. Be economical with the use of water.
   4.Run the hot water slowly ; this will avoid the mixing of hot and cold water in the storage tank.
   5.Set the thermostat of the electrical backup as low as is comfortable.
   
   
   So this what you have to do to maintain your solar cooker as you can see it will require very less cost for maintenance.

The box contains copper tubes  bent in the form of a coil (zig-zag shape). This is done to increase the surface area  for absorbing heat.

Silver is the metal which is used for connecting wires that connect solar cells in a solar panel, due to its large conductivity.

Solar cell use light energy from photons (basic unit of light energy) and convert it to electricity using semiconductor physics.

The total surface area of solar cells on which solar energy is falling = area of each cell $\times$ number of cells $= 4 \times 3000 {cm}^{2} = 1.2  {m}^{2}$
The solar energy falling on 3000 cells 
${Q} _{1} = 1.2 \times 640  J = 768  J$
The energy converted into electric energy $= \displaystyle\frac{20}{100} \times 768 = 153.6  J$
The electric energy generated in one second is $153.6  J$

No On cloudy day solar cooker did not get sufficient amount of sunlight to produce heat energy for cooking.

If we did not adjust the parabolic mirror in solar cooker according the  movement of sun then all ray of sunlight cannot converge at one point .hence  with time we will get lesser amount of heat energy.

Output of solar cell occurs at a constant voltage. Using a group of solar cells in the solar panel increases the amount of current produced. Also, as area is increased more solar energy is captured and more electrical energy is produced.

 Solar energy systems/power plants do not produce air pollution, water poullution, or greenhouse gases. ... However, some toxic materials and chemicals are used to make the photovoltaic (PV) cells that convert sunlight into electricity.

Among all colour black is the best absorber for heat  so outer surface of copper tube is coloured with black.

The internal energy of 1 mole of an ideal gas at temperature $T$ , having $f$ degrees of freedom is given by ,

A solar cell takes solar energy directly from the sunlight and converts it into electrical energy.

A mechanically isolated system is that , which cannot do work on environment and there is no exchange of mass between system and environment but exchange of heat is allowed between system and environment .

A steady state is a state which remains the same over given time period. In heat transfer problems if there is specific rate of heat transfer through medium (for conduction or convection) and if it remains constant at any time then it is said to have steady state heat transfer. In steady state, there may be different temperatures in body at different locations but the temperature at any given point will remain constant. If the temperature at all locations is same then it is uniform.

Macroscopic variable is a measurable quantity used to describe the complete state of the system. The state of a macroscopic system in equilibrium can be described in terms of measurable properties as temperature, pressure, and volume, which are also known as thermodynamic variables.

$Answer:-$ D

Answer is A.

This is a complex process by which organic matter is decomposed by anaerobic bacteria. The decomposition process produces a gaseous byproduct often called biogas, which consists primarily of methane, carbon dioxide, and hydrogen sulfide. It occurs in nature and can be used to produce biogas from biomass in an anaerobic digestor. 
To optimize the digestion process, the biodigester must be kept at a consistent temperature, as rapid changes will upset bacterial activity. In most areas of the United States, digestion vessels require some level of insulation and/or heating. Some installations circulate the coolant from their biogas-powered engines in or around the digester to keep it warm, while others burn part of the biogas to heat the digester. In a properly designed system, heating generally results in an increase in biogas production during colder periods.
Hence, the statement is true.

The dried manure left after the biogas process is even richer than ordinary muck and makes a fantastic organic fertilizer.
Hence, The spent animal dung left after the production of biogas is an excellent manure.

When solar dryer is placed in sunlight then it convert solar energy into heat energy and heat the air inside the solar dryer to a constant temperature.

 Solar dryer is one of the oldest methods of preserving food for later use.  Foods can  be dried in the sun, in an oven or in a food dehydrator by using the right. So we can dried fruits for longer duration of time.

The basic function of a solar dryer is to heat air to a constant temperature with solar energy hence Solar dryer works on the principle of conversion of solar energy into heat energy.

At night there is no sunlight to produce heat energy so we cannot use solar cookers in night.

We use the black coloured box to absorb more heat as black colour is the best absorber for heat among its all colours.

Sun is the ultimate source of energy because all renewable and non renewable sources are ultimately derived form sun.