Explanation:

- "Once in a blue moon" refers to something that happens very infrequently. It's derived from the rare occurrence of two full moons in a calendar month.

- Option 1: Very rarely - This highlights infrequent events, aligning with the idiom's meaning.

- Option 2: Frequently - This suggests regular occurrences, opposite to the meaning of the idiom.

- Option 3: Incessantly - This implies continuous action, not matching the idiom's episodic nature.

- Option 4: Occasionally - This suggests something that happens from time to time but not as infrequent as "very rarely".