Why is answer choice B wrong?

Nevermind, disregard this question

Hi. I have the same question. I was wondering why the correct answer is D rather than B.

Hi, B and D do look somewhat similar- but there are a few key differences. B is only talking about the promises being kept, while D is broader, it is about a promises overall reliability. However, the key difference is that the argument never assumes that a promise made for selfish reasons is never kept. It says that are not to be relied on, which is not at all the same thing as saying that they are never kept. D is saying exactly that.