Answer:
B. 21
Explanation:
The fractions are there to just make the problem seem complicated, but just rewrite it without the fractions.
"What is a common multiple for 7 and 3?"
In this problem it's easy, all you have to do is guess and check.
A: A doesn't work. 7 is not a factor of 18.
B: B works! Both 3 and 7 are factors of 21.
C: C doesn't work. 7 is not a factor of 3.
D: D doesn't work. Neither are a factor of 4.
They only leave us with one solution, B.