Which word correctly completes the sentence?
taller, brett or mac?
a. who's
b. whose

Who's taller, Brett or Mac?
The answer is who's because who's is the contraction for who is. If you can put who is in the blank and have it make sense that is the word you need to use.
Who is taller, Brett or Mac? This makes sense, so now we know that who's is the correct term.
Whose indicates possession. Example: Whose pencil is that? Try replacing whose with who is. Who is pencil is that? This does not make sense, so now we know that the answer is whose.