Answer:
C
Step-by-step explanation
An easy way to figure is to see if all of the numbers in that group can be divided by 6.
In this first one, 9 and 15 are not divisible by 6.
In the second one 6, 12 are multiples ok 6 but there is one more.
In the last one 24 is over the 20 limit.
So, C is correct.
