Hydrogen-Oxygen fuel cells are used in spacecraft to supply:
A. Power for heat and light
B. electrical power
C. Oxygen
D. Water

Hint: A fuel cell belongs to the class of electrochemical cells which do the work of the generation of electrical energy from fuel by an electrochemical process. To produce electrical energy, they require supply of fuel in continuous manner and an oxidizing agent such as Oxygen. Some of the fuels used in fuel cell are carbon based fuels such as biogas and natural gas.

Complete Step by Step Solution:
Let's understand the fuel cell in detail. A fuel cell has a similar setup of electrochemical cells. A fuel cell comprises an anode, cathode and electrolyte. The role of an electrolyte is to the motion of protons.

Now, we will discuss how a fuel cell works. Hydrogen, when undergoing a reaction with oxygen, generates electricity via the use of fuel. In the fuel cell, there is the transfer of oxygen and hydrogen via carbon electrodes into a solution of sodium hydroxide (concentrated form). The reactions at the cathodes and anodes take place as given below.

Reaction at cathode: \[{{\rm{O}}_{\rm{2}}} + 2{{\rm{H}}_{\rm{2}}}{\rm{O}} + 4{e^ - } \to 4{\rm{O}}{{\rm{H}}^ - }\]
Reaction at anode: \[2{{\rm{H}}_{\rm{2}}} + 4{\rm{O}}{{\rm{H}}^ - } \to 4{{\rm{H}}_{\rm{2}}}{\rm{O}} + 4{e^ - }\]

The net cell reaction is : \[2{{\rm{H}}_{\rm{2}}} + {{\rm{O}}_{\rm{2}}} \to 2{{\rm{H}}_{\rm{2}}}{\rm{O}}\]

As we find out, fuel cells do the work of the generation of electricity by the process of electrolysis; therefore, the use of fuel cells in the spacecraft is for the generation of electrical power.
Hence, option B is right.

Note: It is to be noted that the reaction of a fuel cell is a very slow process. To make the reaction take place at a faster rate, the finely divided catalysts namely palladium, and platinum are used. These catalysts are used in the form of finely divided to increase the effective surface area.