Answer: Odd multiple of 3.

Explanation:
Given : The greatest common factor of a number n and 6 is 3.
Then, it is clear that n should be a multiple of 3.
Prime factorization of 6 :

So for n a even number then the greatest common factor will be 6.
Thus , the possible choice for n is a odd multiple of 3.
Hence, the possible whole-number values for n are :
