The correct answer is option 3: Gambusia fish. Gambusia fish, also known as mosquito fish, are widely used to control mosquito populations in ponds and wells. These small fish are known for their voracious appetite for mosquito larvae. When released into ponds and wells, Gambusia fish feed on the mosquito larvae, effectively reducing the mosquito population.

Option 1: Crab - Crabs do not play a significant role in controlling mosquito populations. They primarily feed on other small aquatic organisms such as algae, plankton, and small fish.

Option 2: Dogfish - Dogfish, also known as sharks, are not typically used for mosquito control. They are larger predatory fish that feed on other fish and marine animals.

Option 4: Snail - Snails, specifically certain species like the apple snail, can help control mosquito populations indirectly. They feed on mosquito larvae and their eggs. However, snails alone may not be as effective as Gambusia fish in controlling mosquito populations.

Therefore, option 3: Gambusia fish is the most suitable choice for controlling mosquitoes in ponds and wells.