Where is this from? I’d say staff is uncountable, so I agree with you that it’s either A or B

A is the only natural correct answer. I cannot say that C is grammatically wrong, but it sure is weird. It is difficult to use the plural "staffs", but not impossible. Here is a paragraph using the word "staffs" correctly: "The Department of Education is composed of many agencies. There is the Kindergarten Agency, the Middle School Agency, and the University Agency. Each agency has its own separate staff. Each of these staffs is paid by the Treasury Department." Although (C) is a possible sentence, I cannot imagine that anybody would ever say it. If they did say it, they should change "who" to "that" or "which" because a staff is not a person.

The correct answer is (A) staff members. “Staff” is an uncountable noun, so when referring to individuals, we say “staff members.” Great job on your learning journey! Let me know if you need more help. ☺️