Excretion in Paramecium sp. is by diffusion because

Analyze the question: It asks why a single-celled organism like Paramecium can get rid of waste (such as carbon dioxide and ammonia) by simple diffusion instead of needing a specialised excretory system.

Concept: Diffusion is the movement of particles from a region of high concentration to a region of low concentration. It works well only over short distances. How fast and how efficiently substances diffuse in or out of an organism depends on its surface area to volume (SA:V) ratio.

• Very small organisms have a large SA:V ratio. Every part of the cell is close to the surface, so wastes move out quickly by diffusion.
• Larger organisms have a small SA:V ratio. Diffusion alone is too slow, so they need specialised organs such as kidneys, lungs and gills.

Evaluate the options:

• A. Its habitat is water and moist places. The habitat helps because water is available, but it does not explain why diffusion alone is enough. Many aquatic animals, such as fish, still need specialised excretory organs.
• B. It has simple, small and few internal organs. Paramecium is unicellular, so it has organelles rather than organs. This option is also a description, not the scientific reason.
• C. It has a large surface area to volume ratio. Correct. Its tiny size gives a large SA:V ratio, so wastes can diffuse across the cell membrane fast enough.
• D. It has a large efficient meganucleus. The macronucleus controls metabolic activities, not excretion by diffusion.

Answer: The correct option is C.

Tip: In questions about why a small organism uses diffusion, think "small size, large SA:V ratio." Whenever you see diffusion linked to gas exchange or excretion in unicellular organisms, look for the option mentioning surface area to volume ratio.