The substance reduced in a hydrogen-oxygen fuel cell is oxygen hydrogen water hydrogen peroxide\

The substance which gets reduced in the hydrogen-oxygen fuel cell is oxygen.

Explanation:

A reduced substance undergoes reduction reaction. This is defined as the reaction in which an atom gains electrons.

A fuel cell is defined as the electrochemical cell which converts the chemical energy of a fuel (often used hydrogen) and an oxidizing agent (often used oxygen) into electrical energy via a pair of redox reactions.

The reactions which occur in hydrogen-oxygen fuel cell are:

At cathode:  

At anode:  

As, the oxygen is gaining electrons from the above reaction. Thus, it is undergoing reduction reaction and is getting reduced.

Hence, the substance which gets reduced in the hydrogen-oxygen fuel cell is oxygen.