In hydrogen-oxygen fuel cell, combustion of hydrogen occurs to

A hydrogen-oxygen fuel cell converts the chemical energy of the reaction directly into electrical energy. Although the overall reaction is the same as the combustion of hydrogen,

$$2H_2(g)+O_2(g)\rightarrow 2H_2O(l)$$

the reaction is carried out in separate half-cells so that electrical energy can be obtained.

At the anode, hydrogen undergoes oxidation:

$$2H_2(g)+4OH^-(aq)\rightarrow 4H_2O(l)+4e^-$$

At the cathode, oxygen undergoes reduction:

$$O_2(g)+2H_2O(l)+4e^-\rightarrow 4OH^-(aq)$$

By separating the oxidation and reduction processes, electrons released at the anode cannot directly combine with oxygen. Instead, they flow through an external circuit from the anode to the cathode.

This flow of electrons creates a potential difference between the two electrodes and generates electrical current.

Although some heat and pure water are produced as byproducts, the primary purpose of carrying out the combustion of hydrogen in a fuel cell is to generate electricity by creating a potential difference between the electrodes.

Hence, the correct answer is Option (D): create a potential difference between the two electrodes.