In the following questions, four alternatives are given for the idiom/phrase printed in bold in the sentence. Choose the alternative which best expresses the meaning of the idiom/ phrase as your answer.
The cops were on their toes throughout the day.

Options

A. eager
B. alert
C. harried
D. quick

Answer

B

Explanation

Idiom to keep somebody on their toes means to make sure that somebody is ready to deal with anything that might happen by doing things that they are not expecting: alert.
Look at the sentence :
Surprise visits help to keep the staff on their toes.