The answer is B.
Step-by-step explanation:
First we multiply each numbers by each other, lets start with option A.
1 x 18 = 18
2 x 9 = 18
3 x 6 = 18
4 x ? = ?
as you can see 4 has nothing to multiply by, so next option.
1 x 18 = 18
2 x 9 = 18
3 x 6 = 18
and nothing left over so it is correct.
the last options are obviously not it because they dont have 1 or the number we are finding the factors of (18) and they are always there so the options C & D are incorrect.
Thus, Option A, C, and D are incorrect, B is correct. '
hope this helps. :)