In this sentence, "five-man" is the correct form to describe the panel. "Five-man" is a compound noun used to describe a panel consisting of five individuals. The word "man" in this context is being used as a gender-neutral term to describe a group of people, similar to how "mankind" can refer to all people, not just men.

The other options are not correct because:
A. "Five-man's" is incorrect because the possessive form is not needed in this context.
C. "Five-men's" is incorrect because the plural form "men's" is not appropriate here.
D. "Five-men" is also incorrect because it uses the plural form "men," which is not the correct form for describing the panel.

Therefore, the correct completion of the sentence is: "The students refused to appear before the five-man panel set up to look into their demands."