In many species of Drosophila, flies of any genotype that are reared in a cold environment tend to be larger than flies of the same genotype raised in a warm environment. Is this an adaptation

It is an adaptation.

Explanation:

An adaptation is classified by the mutations seen in a creature in order to survive in a specific climate. If the fly is bigger in the colder climate, it is has adapted to withstand harsher temperatures than that of the warmer climate fly.