The correct option is C.
A Plateau period is a period of time when one experiences little or no progress, during this period, everything is at standstill, that is, stagnant. Looking at the sentence in option C, it can be inferred that the employee is tired of his current position because he has been occupying the same position for a very long time; his career has been stagnant for a long time; he is experiencing a career plateau period.
