what is the GFC of 2

I will assume you are asking for GCF (not GFC)

The greatest common factor of 10 (1,2 x 5)  and 2 (2 x1)  IS 2    so 'a' is correct

                                               of 2 (2x1)  and 24 (1,2,3,4,6,8,24)  IS 2  so 'b' is correct

                                                of 6 (1,2,3,6) and 12(1,2,3,4,6,12) is 6  so 'c' is incorrect

                                               of 4(1,2,4)  and 6 (1,2,3,6)   IS 2 so 'd' is correct

a  and  b  and  d