(b) Activity of respiratory centres is directly proportional to the level of free $CO _2$ in blood.

A normal man, when resting, usually respires about 12-18 times per minute. So, the correct option is 'False'.

The volume of air inspired or expired during normal breath is called tidal volume. It is about 500 mL of air in average young adult man.

Heavy laboured breathing is a sign of abnormal respiration and an increased effort to breathe. Rapid breathing or Tachypnea has seen the breathing rate in adult humans is greater than the normal 20 breaths per minute. This  breathing pattern reduces carbon dioxide in the blood due to increased rate or depth of respiration.

The most significant limiting factor in high mountain regions is the lower air pressure, which makes it more difficult for oxygen to enter our vascular systems resulting into hypoxia. When we travel to high mountain areas, our bodies initially develop inefficient physiological responses. Then there is an increase in breathing and heart rate to as much as double, even while resting. Pulse rate and blood pressure go up sharply as our hearts pump harder to get more oxygen to the cells. These are stressful changes, especially for people with weak heart. Later, a more efficient response normally develops as acclimatization takes place. Additional red blood cells and capillaries are produced to carry more oxygen. The lungs increase in size to facilitate the osmosis of oxygen and carbon dioxide. There is also an increase in the vascular network of muscles which enhances the transfer of gases.

Babies and children normally breathe faster than older children and adults. Newborn to 6 months: 30-60 breaths/min

• The respiratory system represents the organs that are involved in the exchange of oxygen and carbon dioxide between the organism and the environment.
• In animals, a primary layer of cells is formed during embryogenesis, three in number and are differentiated as ectoderm, mesoderm, and endoderm. The tissues produced by these layers give rise to the organs of the body. 
• The respiratory tract with tracheae, bronchi, alveoli of the lungs is developed from the endoderm. The mesoderm forms the mesothelium from which lines the thoracic cavity and external intercostal muscles.
  Hence, the correct answer is D.

The blood pumped by the right ventricle enters the pulmonary artery, whereas the left ventricle pumps blood into the aorta.The deoxygenated blood pumped into the pulmonary artery is passed onto the lungs from where the oxygenated blood is carried by the pulmonary veins into the left atrium. This pathway constitutes the pulmonary circulation. 

Altitude sickness is also known as acute mountain sickness (AMS), altitude illness, hypobaropathy, "the altitude bends" or soroche is a pathological effect of high altitude on humans, caused by acute exposure to low partial pressure of oxygen at high altitude. It commonly occurs above 2,400 metres (8,000 feet). 

The breathing centre in the brain responds to changes in the carbon dioxide concentration of the blood.

Respiration is a process based on chemical reaction and takes place  in all living cells. In this process, energy is released from glucose, for the functioning of our cells. Sugars used as a substrate for respiration comes from the hydrolysis of starch which results in loss of dry matter. 

The main respiratory surface in humans is the alveoli, which are small air sacs branching off from the bronchioles in the lungs. They are one cell thick and provide a moist and extremely large surface area for gas exchange to occur. Capillaries carrying deoxygenated blood from the pulmonary artery run across the alveoli. Due to the enormous number of alveoli (approximately 300 million in each human lung), the surface area of the lung is very large (75 m2). Having such a large surface area increases the amount of gas that can diffuse into and out of the lungs.

Not all cells use oxygen to produce energy. Aerobic cells use oxygen to produce energy. Anaerobic organisms do not require oxygen to produce energy. They obtain energy from metabolism in absence of oxygen. For example, yeasts in absence of oxygen convert sugar into alcohol and produces energy. This energy is then utilized for their survival.

The rate of respiration is highly dependent on concentration of carbon dioxide. The higher the concentration of carbon dioxide, less is the rate of respiration. Less the concentration of oxygen and more is the concentration of carbon dioxide, respiration rate increases.

• The combination of oxygen with haemoglobin is called oxyhaemoglobin and this oxygenated blood is carried away from the lungs through the bloodstream to all the tissues of the body. 
• Carbon monoxide can also bind to haemoglobin but does so about 240 times more tightly than oxygen, forming a compound called carboxyhaemoglobin. 
• This means that if both carbon monoxide and oxygen are inhaled, carbon monoxide will preferentially bind to haemoglobin. 
• This reduces the amount of haemoglobin available to bind to oxygen, so the body and tissues become starved of oxygen.

The characteristics of a respiratory surface are thin walls, a moist inner surface, a huge combined surface area, a rich blood supply each alveolus is sounded by capillaries. Surfaces which dries out easily, thick and easily damaged are not a characteristic of good respiratory surface. 

The respiratory rate or breathing frequency is the rate at which the number of inhalation and exhalation cycles are  taken within a set time.  The typical respiratory rate for a healthy adult at rest is 12–20 breaths per minute. However, the average resting respiratory rates differ with age. Food stimulates the nervous system which further tends to increase respiration rate.

The lack of oxygen or a lowering of the normal level of oxygen in the blood and body fluids is known as anoxemia and is generally experienced when in high altitudes.  

The majority of carbon dioxide molecules (85 percent) are carried as part of the bicarbonate buffer system. In this system, carbon dioxide diffuses into the red blood cells. Carbonic anhydrase (CA) within the red blood cells quickly converts the carbon dioxide into carbonic acid (H$ _2$CO$ _3$). Carbonic acid is an unstable, intermediate molecule that immediately dissociates into bicarbonate ions (HCO$ _3$−) and hydrogen (H+) ions. Since carbon dioxide is quickly converted into bicarbonate ions, this reaction allows for the continued uptake of carbon dioxide into the blood, down its concentration gradient. It also results in the production of H+ ions. If too much H+ is produced, it can alter blood pH. However, hemoglobin binds to the free H+ ions, limiting shifts in pH. The newly-synthesized bicarbonate ion is transported out of the red blood cell into the liquid component of the blood in exchange for a chloride ion (Cl-). This is called the chloride shift. When the blood reaches the lungs, the bicarbonate ion is transported back into the red blood cell in exchange for the chloride ion. The H+ ion dissociates from the hemoglobin and binds to the bicarbonate ion. This produces the carbonic acid intermediate, which is converted back into carbon dioxide through the enzymatic action of CA. The carbon dioxide produced is expelled through the lungs during exhalation. 

Anaerobic respiration takes place in the absence of oxygen. So, oxidation of food does not take place. So, statement c is wrong. Gaseous exchange occurs through leaves that have minute tiny apertures over them called stomata.