In a hydrogenoxygen fuel cell, combustion of hydrogen occurs to (a) generate heat

(b) : Direct conversion of chemical energy to electric energy can be made considerably more efficient (i.e. upto 75%) than the 40% maximum now obtainable through burning of fuel and using the heat to form steam for driving turbines. Furthermore, the water obtained as a byproduct may be used for drinking by the astronauts. 

At anode : 2H_2(g) + 4OH^–_(aq) → 4H₂O_(l) + 4e^– 

At cathode : O_2(g) + 2H₂O_(l) + 4e^– → 4OH^–_(aq) 

2H_2(g) + O_2(g) → 2H₂O_(l)

Correct option(b) create potential difference between the two electrodes

Explanation:

Any cell (like fuel cell), works when potential difference is developed.