The correct answer is answer D ("Slippery slope").
This type of fallacy presents itself when the clear core of a discussion is taken out of proportion by suggesting a possible chain of negative events that could come as a direct consequence of that core element.
In this case, the core element of the discussion is whether or not a school should determine how their students should dress, which is a reasonably small imposition. The argument against it suggests that if we give a school that right, they would be likely to also try imposing what students can say outside of class, which is a wild exaggeration. It's clear how out-of-propotion this argument is as the school would have no way of monitoring students outside school and there's no clear reason to suspect the school wants this level of control anyways.
Looking out for this type of wild exaggerations that try to relate two very different events and disguise them as a cause-and-effect realtionship is the best way of recognizing the slippery slope fallacy.
Hope this helps!