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 makes the author's explanation, based on pedestrians being less careful in crosswalks, seem less likely to be the explanation.

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 why more pedestrians are hit in one place (crosswalks) than any other place at a given time.

This doesn't weaken the argument. It's unclear if this is trying to suggest an alternative explanation, because technically pedestrians underestimating the likelihood of signals to malfunction could just be a sign of their overly strong sense of security in cars following the signals. If so, this answer choice would strengthen the author's claim. But even if this is trying to propose an alternative explanation than pedestrians having an overly strong sense of security, we don't know how often this underestimating of signals occurs, and how much it affects pedestrians being struck by cars.

This strengthens the argument, because it eliminates the alternative hypothesis that drivers are the careless ones rather than pedestrians. This makes it more likely that the reason more pedestrians are struck in crosswalks has to do with the pedestrians themselves, as the author suggests.

This supports the author’s argument, by suggesting that crosswalks and traffic signals might make people less cautious.