When they were first sold, aerosol insecticides were highly effective in killing flies and mosquitoes. now, several decades later, a much smaller proportion of these insects die when sprayed. the reason fewer insects die when they are sprayed is that

a. the original spraying has caused a permanent mutation, giving the insects genetic resistance to the spray.
b. mosquitoes are deliberately adapting themselves to this man-made change in the environment
c. mosquitoes that survive spraying develop an immunity to the insecticided.
c. many mosquitoes today are descendants of mosquitoes with insecticide-resistant characteristicse.
d. none of the listed responses is correct.

a. The original spraying has caused a permanent mutation, giving the insects genetic resistance to the spray.

Explanation:

This is a practical   evidences   of mutation and  variation,  as   mechanisms of evolution.

The insecticide is the selective pressure. As a results of continuous spray,  the parents of the present mosquitoes  must have acquired certain characteristics or traits by  gene mutation which makes them resistant to the selective pressure (insecticides), and therefore survive the spray  in the past.

Therefore when these resistant parents mated, with high reproductive success (leading to large gene pool of these traits,); variation in the genetic composition increases as the resistant gene is passed from generations to generations in the population. Therefore, the population of  present day mosquito with   genetic resistance to insecticides  increases.They  are therefore  naturally selected, and  able to  survive as  the  present day mosquitoes with high resistance to insecticides.