Answer:
A. In a population of bed bugs, variation exists. Some variation provides resistance to the pesticides. Those bed bugs survive to reproduce, passing the resistance on to offspring.
Step-by-step explanation:
Variations are already present among the individuals of sexually reproducing population. The beneficial variations such as resistance to pesticides impart survival and reproductive advantage to the individuals carrying the variations. These variations are favored by natural selection. The bugs carrying the resistance gene reproduce and pass the trait to the offspring.