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)