What is the largest possible remainder when a two-digit number is divided by the sum of its digits?
a) 13
b) 14
c) 15
d) 16
e) 17

The largest possible digit sum = 9 + 9 = 18, so no remainder can be > 17.

Working down from the top:
99/18 → remainder 9
98/17 → remainder 13
89/17 → remainder 4
97/16 → remainder 1
88/16 → remainder 8
79/16 → remainder 15
 
After that the divisor is 15 or less,
so the remainder is 14 or less,
and so the largest remainder is 15,
from 79 = 4 * 16 + 15.