Explanation:
The incorrect statement among the given options is option 'b': The residual air in lungs slightly decreases the efficiency of respiration in mammals.

Reason:
Residual air refers to the air that remains in the lungs after expiration. This air helps to keep the alveoli inflated and prevents them from collapsing completely. It also provides a reservoir of oxygen for the next breath and ensures continuous gas exchange. Therefore, the presence of residual air in the lungs does not decrease the efficiency of respiration in mammals, but rather plays a crucial role in maintaining efficient gas exchange.

Correct Statements:
a) The principal of countercurrent flow facilitates efficient respiration in gills of fishes:
- Countercurrent flow in fish gills refers to the opposite flow of water and blood across the respiratory surface.
- This mechanism allows for efficient exchange of oxygen and carbon dioxide.
- Oxygen-rich water flows across the gill filaments in one direction, while blood flows in the opposite direction through the capillaries.
- As a result, a concentration gradient is maintained along the entire length of the respiratory surface, maximizing the diffusion of oxygen into the bloodstream and the removal of carbon dioxide.
- This countercurrent flow ensures efficient respiration in fishes, allowing them to extract a higher percentage of oxygen from water compared to other organisms.

c) The presence of non-respiratory air sacs increases the efficiency of respiration in birds:
- Birds have a unique respiratory system that includes air sacs.
- These air sacs are connected to the lungs and other parts of the body.
- During respiration, air flows in a unidirectional manner, passing through the lungs and into the air sacs.
- This unidirectional flow allows for efficient gas exchange and ensures a constant supply of oxygen to the tissues.
- The air sacs also act as reservoirs, storing air and allowing for continuous airflow even during the expiration phase.
- This efficient respiratory system enables birds to meet the high oxygen demands required for their active lifestyle, such as flying.

d) In insects, circulating body fluids serve to distribute oxygen to tissues:
- Insects have a highly specialized respiratory system composed of a network of tubes called tracheae.
- These tracheae directly deliver oxygen to the tissues without the need for a circulatory system.
- Oxygen enters the tracheae through tiny openings called spiracles located on the body surface.
- The tracheae branch into smaller tubes called tracheoles, which directly contact the cells and deliver oxygen to them.
- Oxygen diffuses from the tracheoles into the cells, while carbon dioxide produced during cellular respiration diffuses out.
- The circulation of body fluids in insects, such as hemolymph, serves other functions like nutrient transport, waste removal, and hormone distribution, but not oxygen distribution.

In summary, the incorrect statement is option 'b': The residual air in lungs slightly decreases the efficiency of respiration in mammals. The other statements are correct and explain the efficiency of respiration in different organisms.

The correct answer should be option (d)
The correct explain is given already.