Select the most more appropriate option to fill in the blank.

Don’t buckle _____________ the pressure.

under

The most appropriate option to fill in the blank is under.

The phrase "buckle under pressure" means to give in or collapse under stress or difficulty. In this context, it is the most accurate word to describe what you are telling someone not to do. You are telling them not to give in to the pressure, not to collapse under the stress, and to keep going.

The other options are not as appropriate.

• In means to be surrounded by something. It is not the best word to describe what you are telling someone not to do because it does not capture the sense of giving in or collapsing.
• Over means to be in control of something. It is not the best word to describe what you are telling someone not to do because it does not capture the sense of giving in or collapsing.
• Below means to be lower than something. It is not the best word to describe what you are telling someone not to do because it does not capture the sense of giving in or collapsing.