The correct answer is Gambusia fish. Here`s why:

Option 1: Crabs - While they can eat mosquito larvae, they aren`t usually placed in ponds or wells for this purpose.

Option 2: Dogfish - This is a type of shark and is not used in fresh water environments like ponds or wells. They also don`t feed on mosquitoes.

Option 3: Gambusia fish - Known as mosquito fish, Gambusia are small freshwater fish that are renowned for their ability to control mosquito populations. They actively feed on mosquito larvae, inhibiting the growth of these insects. This makes them a natural and effective solution in controlling mosquito problems.

Option 4: Snails - some types of snails can feed on mosquito larvae but generally they are not as effective as Gambusia fish and hence not usually used for this purpose.

In summary, among the given options, the release of Gambusia fish into ponds and wells is the most helpful in controlling the mosquitoes.