Answer:
C. birds that use the trout as a source of food
Step-by-step explanation:
Living organisms feed on one another in their natural environment in order to obtain energy for their various activities. According to this question, overfishing by humans decreases the population of trout living in a particular river.
This means that in that aquatic ecosystem (river), the population of organisms that feed on trout as a source of energy will also decrease. In this case, it is the birds. Hence, birds that use the trout as a source of food.