What you've posted doesn't make sense. You've just written a function definition. This is valid for all x and thus there is no largest value of x for which this is true.

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

thus

(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.