How did insects become resistant to DDT

A. Some were already resistant and were the survivors that could reproduce.

B. All insects already had genetic resistance to DDT and it took effect once exposed.

C. The gametes of all insects exposed to DDT were mutated for the offspring to be resistant.

D. All surviving insects eventually developed resistance after multiple exposures to DDT.

A. Some were already resistant and were the survivors that could reproduce

Explanation:

Both resistant and susceptible insects will be exposed to DDT, but according to the evolution theory, it is expected that those insects that are resistant to the insecticide will be selected by natural selection. In consequence, the resistant individuals will survive to reproduce in the next generation.  

A.Some were already resistant and were the survivors that could reproduce

Explanation:

Insects are known for their ability to develop resistance to insecticides. Currently there are insects resistant to every synthetic chemical insecticide used. There are many factors to developing resistance. In a population of insects there may be a few individuals that carry the genes for resistance. These genes arise from genetic mutations that are rare.  Upon exposure to insecticides, insects that do not carry the resistance genes die, thus allowing the individuals with the resistance genes to survive and reproduce, creating more resistant insects. With every generation the number of resistant insects increase.

b is dominant over the allele for the white fur.