Corals live near the ocean surface in some warm regions of the world. the corals provide a home for algae. the algae, in turn, produce food chat is used by the corals. which of these is a biotic factor in this ecosystem?
a.. the temperature of the water
b. the salt content of the water
c. the size of the algae population .
d. the amount of sunlight that reaches the coral

The correct answer is C. the size of the algae population .

Explanation:

An ecosystem is made up of biotic and abiotic factors. Biotic factors are the living components of an ecosystem that impact the ecosystem and abiotic factors are the non- living components that have their own impact on the ecosystem.

So according to the definition of biotic factor algae is the biotic factor in this question because algae is a living component of the ecosystem that influences the ecosystem by providing food to the corals.  

So by providing food algae supports the coral's ecosystem and coral provides a home to algae to grow. In the ecosystem, both biotic and abiotic factors interact with each other.

So the correct answer is C. the size of the algae population.