Fill in the blank with an appropriate idiom. The teacher was so impressed with the student's presentation that she gave him a __________.

The idiom "pat on the back" means to praise or congratulate someone for their good work. In this sentence, the teacher is praising the student for their presentation, so the idiom "pat on the back" is the most appropriate option. The other options are not as appropriate in this context. A "kiss of death" is a negative thing, so it would not make sense for the teacher to give the student a kiss of death for their presentation. A "green light" means to give permission, but it would not make sense for the teacher to give the student a green light for their presentation, since they already gave them a pat on the back. A "red flag" is a warning sign, so it would not make sense for the teacher to give the student a red flag for their presentation. A "white elephant" is a possession that is useless or unwanted, so it would not make sense for the teacher to give the student a white elephant for their presentation. Therefore, the correct answer is "pat on the back."