Which substance should a student apply to the skin if he or she gets splashed with an acid?

First of all, you should be wearing proper PPE when handling acid, including a lab coat, large viton or butyl rubber gloves over nitrile gloves, apron, safety eye goggles (goggles!! not just safety glasses with an eyeshield), and face shield. You should be working in a fume hood with the sash closed in front of you as much as possible. This combination of engineering controls and PPE will help avoid a lot of big problems.