Everyone laughed when he said he sees his old friend once in a blue moon.What does the underlined idiom mean?

Explanation

The idiom 'once in a blue moon' means something that happens very rarely. In this context, it indicates that the person sees their old friend infrequently. The other options suggest regularity or frequency that contradicts the idiomatic expression. 'Very frequently' (Option A) and 'Every month' (Option C) imply regular encounters, while 'Only in summer' (Option D) suggests a specific season, which does not reflect the rarity indicated by the idiom.