Final answer:
In an area where birds are present but parasitoid wasps are absent, disruptive selection would most likely affect gall size.
Step-by-step explanation:
The correct answer is A. disruptive selection. When gall flies occur in an area with birds but no parasitoid wasps, disruptive selection is expected to affect gall size. Disruptive selection occurs when extreme phenotypes have a higher fitness than individuals with intermediate phenotypes, leading to the selection of both large and small gall sizes.