When they were first sold, certain insecticides were highly effective in killing mosquitoes. today, dozens of mosquito generations later, a much smaller proportion of these insects die when sprayed with the same chemicals. fewer insects are killed today because
a. mosquitoes are deliberately adapting themselves to this man-made change in the environment
b. mosquitoes that survive spraying develop an immunity to the insecticide
c. many mosquitoes today are descendants of mosquitoes with insecticide-resistant characteristics
d. the original spraying has caused a permanent mutation, giving the insects genetic resistance to the spray

C. many mosquitoes today are descendants of mosquitoes with insecticide-resistant characteristics

Explanation:

When insecticides were first sold, they where highly effective in killing mosquitoes but few mosquitoes will definitely survived. If these mosquitoes that survived mate together, they  will produce hybrid offspring that possess characteristics that are  insecticide-resistant. The more  insecticide-resistant mosquitoes that breed together in subsequent generations, the more permanent mutant hybrid offspring will be produced which will eventually make them to become resistant to the use of insecticides over them.

i think the answer is b.