Within a decade of the introduction of a new insecticide, nearly all of the descendants of the target insects are resistant to the usual-sized dose. what is the most likely explanation for this change in susceptibility to the insecticide?

The most likely explanation for this phenomena is that this is a result of natural selection.

What actually happened?
When the insecticide was introduced, it killed all of the organisms that were vulnerable to it. However, there was a portion of the population that was immune. Since the immune individuals were the only ones that have survived and left offspring, after some generations in the population of this organism there were only their descendants that have the resistance to the insecticide.