i'm not 100% sure, but i personally would go with A, as the character seems to be very upset and frustrated.
The correct option is C.
A plateau period refers to a period of time in which there is little or no growth or decline, everything is just stagnant. Looking at the sentence given in option C, it can be seen that the person been talked about is tired of his current position, which means that, he has been in the same position for a very long time; that is career stagnancy.
They think he's overreacting because just a few days ago, he was in love with a different girl, and quickly changed his mind when he met Rosaline. They are certain that he will get over it soon, and he's being ridiculous.