The correct answer is D. Mangrove tree crab.
In the given food chain of a mangrove community, green algae are producers, fish is primary consumer as they feed on green algae, mangrove tree crab is secondary consumer feeding on fish, mangrove water snake is tertiary consumer which feeds of mangrove tree crabs and finally American crocodile is top consumer which feeds on snakes. In this food relationship, if the number of mangrove water snake increase, the population of mangrove tree crab will decrease as crab would be eaten by more snakes. But in the long the decrease mangrove tree crab, the population of mangrove water snake will also decrease in want of food.