One of the main advantages of hydrogen as a fuel is that:

a) plants can economically produce the hydrogen needed
b) it exists as a free gas
c) it can be economically supplied by the world's ocean
d) the only product of hydrogen combustion is water
e) it contains a large amount of energy per unit volume of hydrogen gas

The correct answer is option D.

Explanation:

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 we can see that reaction of hydrogen gas with oxygen gas gives us the water as a product which can be used for other things and it also doesn't pollutes environment.