Can someone please explain why A is incorrect?

@mprezzy, happy to help! Answer choice (A) is incorrect because it is irrelevant. We are not concerned with what exact proportion of employees eat regularly at the cafeteria. We are only concerned with the fact that, of those people that did eat at the cafeteria, more chose the mushroom casserole than any other main dish. Whether 10% or 90% of employees eat regularly at the cafeteria does not change this fact. Does that make sense? Please let us know if you have any other questions!