The author concludes that accidents that result from cars going around railroad crossing gates are not the fault of the railroad company. This is because the drivers of those cars are adults who should know that they shouldn’t go around railroad crossing gates.

Note that it’s important to translate the conclusion — “this is a mistake” — into “the railroad company is not (at all) at fault.” If you don’t realize that the conclusion is asserting that the railroad company is 0% at fault, you’ll struggle with this question.

We’re trying to prove that the railroad company is not at fault — not even partially at fault. But the premise doesn’t establish anything about who is or isn’t at fault. So, at a minimum, we want an answer that mentions something about fault.

Moreover, the answer, in connection with the premises, must establish that the railroad company bears no fault at all. If the answer allows the railroad company to possibly bear partial fault, it’s not correct.

Here’s an example answer that could make the argument valid:

If an accident could have been avoided by an adult who should know better than to act in a way that led to the accident, then nobody else is at fault except the adult.