As this is a question about finches, I figured I should chime in
The big issue I think you're having here is focusing on conditional reasoning in the stimulus, when it's actually using causal reasoning chains, the use of the word "because" being the big giveaway here. What the confusing question stem is ultimately asking for is an answer choice that will eliminate a potential causal objection to the argument (alternate cause, reverse causation, cause w/o effect or vice versa) as a Defender Assumption. The correct answer choice here, (B), does that by eliminating a potential situation where a cause (abundance of large, hard seeds during a rainy period) could exist while its purported effect (greater numbers of large finches) doesn't, which would directly attack the argument's ultimate conclusion of climate being a cause for the population size of the different kinds of finches.
Hope this clears things up!