Correct option is C
Given Idiom: Once in a blue moon: This idiom means something that happens very rarely. (बहुत कम बार होने वाला)
Example: He visits his hometown once in a blue moon.
Other related idioms and their meanings:
Once in a lifetime – Extremely rare occasion.
Hardly ever – Almost never.
Every now and then – Sometimes.
Blue moon – A rare event.