The greatest common factor (or GCF) is, as the name says, the greatest factor a set of numbers have in common. We can find this by listing out all the possible factors (multiples) of each number:
For 28:
1, 2, 4, 7, 14, 28
For 42
1, 2, 3, 6, 7, 21, 42
(note you don't need to list out all the factors, I just did it for visual purposes)
As you can see, 28 and 42 have 3 factors in common, with 7 being the greatest.
Therefore, the answer would be C: the GCF is 7.
Hope this helps! :)
