Divisors of 18 are 1, 2, 3, 6, 9, 18.
Divisors of 24 are 1, 2, 3, 4, 6, 8, 12, 24.
The largest number on both lists is 6. This is the GCF of 18 and 24.
_____
a) No. The number present must be a divisor of 6.
b) Yes. The number present is a divisor of 6.
c) The number present must be a divisor of 6.
d) Possible numbers present are 1, 2, 3, 6.