Concept:- The question tests on abilities to eliminate options
Solution:
Use the options here and eliminate them.
A.9 cannot be expressed as sum of multiple of 3 and 4
B,C,D-Similarly 10, 12 or 13 also cannot be expressed as sum of multiple of 3 and 4.
Only 11 can be expressed a sum of multiples of 3 and 4.
11= 3+ 2(4)
We need 1/3 of boys, 1/4th of girls. and number of boys and girls have to be integers.
11 is the only option that satisfies the situation . So,(option e)