The best way to answer this question is by drawing an image of the carton with dimensions listed...
Now arrive upon the diameter of the ball which is 2*2 = 4. Height given is 12...
The maximum number of balls that can be placed height wise is only 1 in case of any dimension of the carton... 12 * 2 = 24 that is greater than 14, 16 and 20. Hence consider the height to be as 14, and consider the length and width as 20 and 16 respectively.
This means along the length the number of balls that can fit are 20/4 = 5 and along the breadth the number will be 16/4 = 4. The total number of balls hence will be 5*4 = 20.
C the right answer !