The release of which one of the following into ponds and wells helps in controlling the mosquitoes?

Correct answer: Gambusia fish (option 3). Small, surface‑feeding larvivorous fishes such as Gambusia affinis have been widely used to control mosquito larvae in ponds and other standing water bodies because they preferentially feed at the surface and breed in shallow habitats where mosquito larvae occur; they are hardy and, once established, can persist and reduce larval densities over time [1]. National and WHO guidance highlights Gambusia (and similar species like Poecilia) as effective biological control agents that target larvae as an environmentally preferable alternative to chemical insecticides, suitable where breeding sites are well defined [2]. Crabs, dogfish and snails are not standard larvivorous agents used for mosquito control.

Sources

1. [1] https://ncvbdc.mohfw.gov.in/Doc/Guidelines%20for%20use%20of%20larvivorus%20fish.pdf
2. [2] https://applications.emro.who.int/dsaf/dsa205.pdf