Water pollution is the condition in which there is an increase in the concentration of the undesirable compounds in the water resources. The pollutants are the organic compounds. The microbes use up the oxygen which is present in the water bodies for the degradation of the organic matter in the pollutants. This lowers the amount of oxygen which is available for the aquatic plants and animals.

Preservation refers to the measures taken to keep living things in their original balanced state. Conservation refers to the careful and systematic use of natural resources without causing harm and death to living organisms in the environment. Recycling of waste, reducing the use of non-renewable natural resources and alternative source are the measures to maintain the natural resources without harming the resources and environment. Thus, the correct answer is option D.

Natural resource management refers to the management of natural resources such as land, water, soil,  plants and animals, with a particular focus on how management affects the quality of life for both present  and future generations. This includes all the above a,b & c points hence d option is the right answer.

Biogas is a flammable gas that is produced from the fermentation or degradation of biomass in biogas plants. Biogas typically refers to a gas produced by the biological breakdown (degradation) of organic matter in the absence of oxygen. whereas the breakdown of organic matter, usually by bacterial action, resulting in the formation of other substances of less complex constitution with the evolution of ammonia is known as putrification.

Compact fluorescent lamps (CFL) are the bulbs which are generally used to save the electricity. These bulbs use and convert only 1/5$^{th}$ of the electricity into light. There is consumption of less electricity. This helps in reduction of about 6000 megawatts of electricity per year. 

Nonrenewable resources are resources that are not easily replenished by the environment. Thus coal and  minerals are examples of non renewable resources.

India is ranked 5th largest wind producing country in the world. The largest wind energy farm is in Muppandal, Tamilnadu having the capacity of 1500MW wind production. So, the correct option is C.

CFL is compact fluorescent light. Compact fluorescent light bulbs are used in place of incandescent bulb because they use about 1/7$^{th}$ of the power used by the incandescent bulb. Hence, they are highly efficient in saving electricity.

Oil is a mixture of molecules that mainly contain carbon and hydrogen. When it burns it also produces CO$ _2$, but 25% less than coal for the same amount of energy (because the combustion of hydrogen gives energy but generates only water). 

Energy generated by using wind, hydrogen, tides, solar, geothermal heat, and biomass is known as non-conventional energy. These are the inexhaustible non-conventional resource. Hence, these are the best choice for energy production.

Photosynthesis is the process by which green plants and certain other organisms transform light energy into chemical energy. Photosynthesis in green plants harnesses the energy of sunlight to convert carbon dioxide, water, and minerals into organic compounds and gaseous oxygen. Hence, planting green trees is an easy way to conserve solar energy.

About 20% of the world oxygen is produced in the rainforests. amazon They are described as the  'lungs of the planet' because they provide the essential world environmental service of continuously recycling $CO _$ into $O _2$.

• Energy plantation is the practice of planting trees, purely for their use as fuel. Terrestrial biomass i.e., the wood plants have been used for a long time to generate fire for cooking and other purposes.
• Energy plantation is an Eco-friendly method or environmental friendly as this method does not cause any harm to the environment.
  
• Hence the Eco-friendly method is Energy plantation.
• So, the correct answer is 'Energy plantation'.

Eco-friendly activities in daily life:

• Separation of biodegradable and non-biodegradable substances.
  
• Gardening
  
• Use of gunny bags/paper bags in place of polythene/plastic bags.
  
• Using public transport for commuting, walking or using cycle for short distances.
  
• Harvesting rainwater.
  
• Switching off light/ fans in unoccupied rooms.
  
• Use of compost and vermicompost in place of fertilizers.