The correct option is (D): 65.
From the given question, we can observe that in each case 5 students are being left out.
So, the number of students in the school will be
LCM of (7, 10 and 15) × k + 5 = 210k + 5
Also, it is given that the number of students in the school is more than 500.
So, if k = 1, number of students = 215 (not possible)
If k = 2, number of students = 420 + 5 = 425 (not possible)
If k = 3, number of students = 630 + 5 = 635 (possible)
After simplifying:
average =80.