The idiom "Once in a blue moon" is used to describe an event that occurs very infrequently. To choose the correct meaning, let's analyze the options:
Based on the analysis, the correct meaning of the idiom "Once in a blue moon" is Rarely.
Rearrange the following parts to form a meaningful and grammatically correct sentence:
P. a healthy diet and regular exercise
Q. are important habits
R. that help maintain good physical and mental health
S. especially in today's busy world