This undermines the argument by providing an alternative explanation for why more pedestrians get hit in crosswalks. It's just a question of frequency: crosswalks are where the vast majority of pedestrians cross! If this is true, and very few pedestrians cross outside crosswalks, then it's not surprising that more pedestrians who are struck by cars happen to be in crosswalks. This phenomenon doesn't have to be the result of pedestrians somehow being less careful in crosswalks.

Knowing the overall trend in the number of pedestrians getting hit by cars everywhere over time doesn't help us weaken this claim. We're interested in explanations for why more pedestrians are hit in one place (crosswalks) than any other place at a given time. This answer choice could perfectly well be true and the author's hypothesis still be correct.

This doesn't weaken the argument. Even if pedestrians trusting that signals will work is technically different from pedestrians trusting that cars will follow signals, both things could be true, and we don't know how often this overestimating of signals occurs and how much it affects pedestrians being struck by cars. So even if this answer choice were true, it wouldn't provide an alternative explanation for the observed correlation, so it wouldn't undermine the author's hypothesis.

This answer choice is tricky, because it might seem like drivers' increased carefulness around crosswalks might somehow make up for pedestrians' increased carelessness, meaning that pedestrians' carelessness can't explain the increased frequency of accidents. But that would be a big assumption about the relative levels of carefulness versus carelessness, and it would leave us with no way to explain the phenomenon: if pedestrians aren't being too careless, and drivers are being more careful, why are more pedestrians struck in crosswalks than elsewhere?

So it's crucial to see that this answer choice is actually perfectly consistent with the author's hypothesis. If we know that drivers are being extra careful around crosswalks, then it seems plausible that accidents occur because pedestrians are overly careless around crosswalks. We've actually strengthened the author's argument by ruling out the alternative hypothesis that drivers, not pedestrians, are careless around crosswalks and responsible for accidents.

This doesn't weaken the argument. It states a general principle, which suggests that crosswalks and traffic signals might well make pedestrians less cautious — but that doesn't tell us whether this lack of caution is the reason for the increased frequency of accidents in crosswalks or not.