The hyphen in "one-time winner" is used because:

a. time modifies one winner

b. one is a prefix

c. one-time is a number

d. one and time modify winner

The correct answer is D. one and time modify winner.

A is incorrect because time is the word being modified (described), and not the other way round. B is incorrect because one is a number, and not a prefix. C is incorrect because one-time is an adjective, and only one is a number.

That leaves us with D as the correct answer - both words, one and time, modify the word winner - and because it is a compound adjective, it has to have a hyphen.