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I was having trouble understanding how B was correct instead of C and this was my thought process to reason it out. Hope this helps someone!

Okay, so what we have to do for weakening questions like these is take the premises to be true, look at the author's conclusion, and figure out how we can damage the bridge that the author used to get there. There are often assumptions made by the author, and identifying those is one of the ways that you can find some clear lines of attack.

In this question, the author knows that ocean sticklebacks have armor to protect them from their predators, while lake sticklebacks do not. We also know that armor limits the speed of a stickleback's growth.

From this, the author takes a leap to conclude that the lake sticklebacks have a larger size because it is a better defense against their predators than having armor would be.

What assumptions does the author make?

He assumes that their discrepancy in size is causally related to the predators that they each encounter in their respective habitats. Just because ocean stickleback have armor to defend against their specific predators doesn't mean that lake stickleback have a large size for the same reason. There could be an alternative explanation.

That is what B provides. It suggests that the size discrepancy is due to the lake stickeback having colder winters.

I initially chose C, but C is wrong because it does not outright undermine the author's argument without requiring you to make some leaps yourself.

Knowing that lake sticklebacks are preyed upon by predatory insects more than by larger fish does not necessarily undermine the argument. It adds some much-needed context, but in order for this to be the right answer, we would have had to know from the stimulus that armor is a better defense against predatory insects than is a large size. This answer choice wants us to dip into our own understanding of the world and what we think is likely to be true, but if it is not corroborated by the stimulus, then we cannot use it to weaken the author's argument. Then, we would make the same mistake that the author does in making unwarranted assumptions.

It is important to note that B is not a perfect weakening answer because it could be that the large size is important for cold winters and equally important for defending against the lake stickleback's predators. In a back-and-forth between the defense and prosecution in a courtroom, the defense (assuming the defense is the author and we are the prosecution) would definitely bring this up. However, for an answer to be correct, we do not need to completely contradict or disprove the argument. B momentarily casts doubt on the author's conclusion by pointing out an alternative explanation that they may not have taken into account. Thus, it is the correct answer for this question.