Some large fish allow smaller first to swim inside of their mouths without eating the smaller ones. The smaller fish get to feed inside the larger fish's mouth, and the larger fish gets parasites removed from its teeth. This is an example of:

A. reciprocal altruism

B. kin selection

C. audience inhibition

D. the cost-reward model

The correct answer is option A. "reciprocal altruism".

Explanation:

Reciprocal altruism is an evolutionary adaptation at which an organism behaves in a manner that allows other organism to get benefited, expecting that this organism will return the favor. The behavior of the large fishes allowing that small fishes enter their mouths is an example of reciprocal altruism. At first, the large fish allows the small fish to be benefited from it by providing food and protection, but later the smaller fish returns the favor by removing the parasites of the large fish teeth.

A

Explanation:

When an organisms tentatively reduces its own fitness, but rather increasing another organism fitness (the host) with the notion that, the host will carry out reciprocal gesture in near future is called A reciprocal altruism. It was coined by Rober Trivers.

The present scenario is an example of Cleaning Symbiosis. Since the smaller fish gets necessary nutrients from the larger fish, with the reciprocal act of cleaning its mouths, the smaller fishes ensured good health or lack of infections through removal of parasite, in a symbiotic relationship. While enjoying good nutrient in return.