What are the common factors that greg found?

The answer is:  1, 2, 7, 14 .

Explanation:

FIrst, list all the factors of 42 (positive integers):

1, 42
2, 21, 
3, 14,
6, 7

     Then list all the factors of 28 (positive integers):

     1, 28
     2, 14,
     4, 7

        Now, list all numbers that occur for BOTH the factors of "28" AND the factors of "42"

        1, 2, 7, 14

      The answer is:  1, 2, 7, 14 .

Ithink the answer is a