The GCF is the number that is a factor of both (or all) of the numbers of a given set and which is the biggest factor of this kind (often the smaller number of the two)
7 and 14 have a greatest common factor of 7.
28 and 7 also have a GCF of 7, so the first two responses fit.
21 and 3 have a GCF of 3, so that isn't correct.
7 and 1 have a GCF of 1, so that's incorrect as well.
