Which of the following adjustments must be made behind the curtain?

The adjustment of back slaps is the correct choice as it refers to a practice that occurs behind the curtain in powerlifting competitions. This action typically involves coaches or handlers giving physical encouragement to the lifter, which can include back slapping for motivation and reassurance before they go out to perform their lifts. This support is crucial for the athlete's mental preparation and can help in developing confidence.

The other adjustments, while important in the context of competition, should not be performed behind the curtain. Adjustments of belts and wrist wraps are usually done in a more open environment where the lifter can be seen, ensuring they are performed correctly and safely. Setting the platform involves the technical setup which is done before the lifting begins and is therefore an element that takes place in the competition area rather than behind the curtain.

In summary, back slaps take place behind the curtain as a form of psychological support, while the others are related to practical adjustments or technical arrangements made in the open lifting area.