Step-by-step explanation:
I think the food shortage (in this case of seeds) caused the changes in the finch population and the average of the beak size during the drought since plants were not producing new seeds, so the smallest-beaked finches had no way to feed themselves, the small seeds ran out very quickly and there were only large seeds that the smallest-beaked finches could not eat, since they were too big for them, that is why they started to disappear, which at the same time increased the average beak depth because only large-beaked finches were left.