Explanation:

- Once in a Blue Moon is an idiom used to describe something that happens very infrequently.

- Option 1: romantic moment - This suggests a special or memorable time, which isn't related to frequency.

- Option 2: occasionally - This means happening from time to time, but not very rarely.

- Option 3: rarely - This means something doesn't happen often, which aligns with the meaning of the idiom.

- Option 4: often - This implies something happens frequently, which is the opposite of what the idiom signifies.