A number system with base X has X different digits ranging from 0 to (X-1).
As in base 10, we have 10 distinct digits ranging from 0-9 whereas in base 2 (binary) we have 2 distinct digits i.e. 0 and 1.
So for base 99, we will have 99 distinct digits ranging from 0 to 98.
That gives X = 99 and Y = 98.
b) is correct.