Further, if you meant the largest value of x such that f(x)=C, where C is some constant, there is still no largest x because f(x) is periodic and will be equal to C infinitely many times as x grows without bound.
Do you mean what is the maximum value that f(x) attains?
The way to solve this is to note that
(there is a formula for but it doesn't matter here)
This clearly attains a maximum value of
This matches your answer (d) and so is probably what is meant.
If this is really the way the question was worded I would have a chat w/your teacher. The question sucks.