The idiomatic expression, Once in a blue moon, means

Correct answer: rarely

Explanation: The idiom ‘once in a blue moon’ means something that happens very rarely or almost never. It refers to the rare occurrence of two full moons in a month, called a blue moon. rarely ✅ – correct meaning. frequently ❌ – opposite meaning. regularly ❌ – opposite meaning. never ❌ – too extreme; blue moon events do occur, though rarely. ✔ Example: He visits us only once in a blue moon. ✅