I take that the insects remain constant no matter what happens to the frogs (which the frogs eat presumably). So a constant food supply for the frogs is not the problem.
The line for the alligators increases over time. It their numbers increase, the frogs are in trouble. The alligators will pursue lunch with determined single mindedness and there are more of them around.
So the frogs should decrease. Their natural enemy is the alligator and alligators won't go after insects. It's not worth their time.
A is the only answer you can choose.