Explanation:

- Once in a blue moon means something happens very infrequently or rarely. It refers to a rare event, like when a second full moon appears in a calendar month.

- Option 1: Occasionally: This would mean something happens now and then, but more frequently than "rarely."

- Option 2: Continuously: This means something happens without stopping. It's the opposite of what the idiom describes.

- Option 3: Rarely: This means something happens very seldom, aligning perfectly with the meaning of the idiom.

- Option 4: Often: This means something happens frequently, which is not what "once in a blue moon" suggests.