The idiom 'At the drop of a hat' means to do something immediately without any delay or hesitation. This phrase is often used to describe actions that are undertaken promptly and spontaneously. For instance, if someone is willing to help you at the drop of a hat, they are ready to assist you right away, without having to think about it.
When examining the given options:
Therefore, the correct answer is To do something immediately.
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