The Following Experiment Is Used for the Corresponding Question

The following experiment is used for the corresponding question.
A researcher discovered a species of moth that lays its eggs on oak trees. Eggs are laid at two distinct times of the year: early in spring when the oak trees are flowering and in midsummer when flowering is past. Caterpillars from eggs that hatch in spring feed on oak flowers and look like oak flowers. But caterpillars that hatch in summer feed on oak leaves and look like oak twigs.
How does the same population of moths produce such different-looking caterpillars on the same trees? To answer this question, the biologist caught many female moths from the same population and collected their eggs. He put at least one egg from each female into eight identical cups. The eggs hatched, and at least two larvae from each female were maintained in one of the four temperature and light conditions listed below.

In each of the four environments, one of the caterpillars was fed oak flowers, the other oak leaves. Thus, there were a total of eight treatment groups (4 environments × 2 diets) .
Which of the following is a testable hypothesis that would explain the differences in caterpillar appearance observed in this population?

A) The longer day lengths of summer trigger the development of twig-like caterpillars.
B) Winter causes ugly caterpillar and trees.
C) Differences in air pressure, due to differences in elevation, trigger the development of different types of caterpillars.
D) Differences in diet trigger the development of different types of caterpillars.