Is it D - 300?
For maximum # of boxes, you should align them within the carton so that the dimension of the carton is divisible by the dimension of the box (ie, no extra room is left). Therefore, you would align the side of the box with length 5 with the side of the carton of length 25, the side of the box with length 6 with the side of the carton of length 60, and the side of the box with length 7 with the side of the carton of length 42. This would allow for 25/5 = 5 boxes along one dimension, 60/6 = 10 along another, and 42/7 = 6 along the other.
Therefore we can fit 5*10*6 = 300 boxes in the carton.
(Another way to look at is would be to find the volume of the carton (25*42*60 = 63000) and divide by the volume of a box (5*6*7 = 210). 63000/210 = 300. Then since we know the the boxes can be packed tightly without leaving extra room, we know 300 is the answer.