Final answer:
When mosquitofish are introduced to the pond, the direct effect on the population of mosquito larvae is an increase in deaths.
Step-by-step explanation:
When mosquitofish are introduced to the pond, there would be a direct effect on the population of mosquito larvae. The correct option that describes this effect is (D) deaths increase. Mosquitofish are natural predators of mosquito larvae and would consume them, leading to an increase in predation and a decrease in the population of mosquito larvae.