I believe that there are two reasons for why answer choice A is incorrect.
1) Just because the wolves prevent other predators from coming to the area, this doesn't explain why the moose population increased even after the wolves were introduced. The whole point of introducing the wolves to the island was to decrease the moose population.
2) I believe it is irrelevant, in this case, to point out that wolves discourage other predators from coming to the area simply because the area is an island. By using our common sense, the wolves themselves had to be artificially introduced because it is an island, so how would any other non-amphibian get there? Do any amphibian predators hunt moose? This is definitely getting too detailed for our analysis. But the idea is that even if wolves discourage other predators from coming to the area, the point is that they couldn't come even if they wanted to.
Regardless of the second point, I believe that the first reason is enough to show why this answer choice is incorrect, because even if other predators do not join the hunt, we did not explain how the introduction of wolves can coincide with an increased moose population.
Hope this helps, please let me know if you have any other questions.