is the lowest common multiple (LCM) of 3 and 7.

The correct option is B 21
Let us first list down the multiples of 3 and 7.

Multiples of 3 are 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, ... and so on.

Multiples of 7 are 7, 14, 21, 28, 35, 42, 49, 56, 63, 70, ... and so on.

Common multiples of 3 and 7 are 21, 42, ... and so on.

Since 21 is the smallest of them, it is the least common multiple (LCM) of 3 and 7.