The 'children' in this question would also be categorized as male or female, so if you were to create all of the potential 'categories of people', they would be...
Adult Male, Adult Female, Child Male and Child Female
That level of detail isn't required in this question though. The information in the prompt clues us in to the idea that we're dealing with MULTIPLES....
There were 5 TIMES as many Females as Males. So F = 5M, and the total number of people can be written as M + F = M + 5M = 6M. Thus, the TOTAL must be a multiple of 6.
Also, there were 3 TIMES as many Adults as Children. So A = 3C and the total number of people can also be written as A + C = 3C + C = 4C. Thus, the TOTAL must also be a multiple of 4.
There's only one answer that does NOT fit BOTH of those multiples...
Final Answer:
GMAT assassins aren't born, they're made,
Rich