Solution

Gambusia is a large genus of freshwater fish though some species are found in brackish or saltwater habitats. There are two species, Gambusia affinis and Gambusia holbrooki. They can feed on mosquito larvae. As a result, they are introduced to the pond so that they can eat up larvae of mosquitoes, which spread various diseases like that of malaria, yellow fever etc. and thereby help in controlling it's spread. Hence, option C is correct.