B is the best answer. Just like the previous question, a person who is strait-laced is someone who is very unbending like a narrow strait. It does not refer to being "the straight man." And it has a hyphen because it is one adjective--the "strait" and the "laced" have to be hooked together to apply this adjective to a person. I have seen "straight-laced" as a variant before, but variants are secondary to the main entries, and I have actually also seen "straitlaced" as an acceptable way to spell it; that's why I didn't use it as a choice, since that would mean there are two right answers.

Please see this page for my source if you're interested.