A viviparous fish is-
A. Labeo
B. Clarias
C. Gambusia
D. Exocoetus

Hint: A large genus of this fish found primarily in freshwater environments has been shown to be useful in curbing mosquito growth, as large quantities of other insect larvae and algae can be eaten to support your pond in various ways.

Complete answer:
Option A is incorrect. In the Cyprinidae family, Labeo is a genus of carps. They are present in the tropics and subtropics of Africa and Asia in freshwater habitats. Labeo rohita is one of India's traditional fresh-water bony fish, usually referred to as the Rohu fish. Oviparous fish such as Labeo (Rohu), Catla (Katla), Clarias (Magur) are most of the bony fishes.
Option B is incorrect. Clarias is a genus of catfish (order Siluriformes) of the air breathing catfish family Clariidae. The name comes from the Greek chloros, which means lively, in reference to the fish's ability to survive out of water for a long time. These fish have slender bodies, a flat, bony head, and four pairs of barbels in a large, terminal mouth. They are oviparous (egg-laying fish), as well.
Option C is correct. Gambusia is a large genus in the Poeciliidae family (order Cyprinodontiformes) of viviparous fish. There are over 40 species in Gambusia, most of which are found mainly in freshwater environments, although some species may be found in brackish or saltwater environments as well. They have demonstrated their ability to curb mosquito growth, and if they are honestly incorporated with the malaria control programme, malaria could be fully eradicated like small pox.
Option D is incorrect. Exocoetus is a flying fish genus and is a marine bony fish as well. Cycloid scales surround the body. The mouth is wide and the jaws are teeth-bearing. Like the ventral lobe, the tail has hypobaric fins. There are also fish that are oviparous.

So, the correct answer is Option C.

Note: In Australia, Gambusia is considered to be invasive pests where they pose a threat to indigenous fish and frog populations and little evidence suggests that they have managed mosquito populations or diseases spread by mosquitoes. In many areas of the world, they were directly introduced into habitats as a biocontrol to lower mosquito populations, which in turn impacted many other species in each different bioregion negatively.