Which common characteristics makes both batteries and fuel cells practical to use?

Both the batteries and fuel cells are portable resources, which transforms the chemical energy into electrical energy.  

A battery will produce electricity from the energy it has accumulated inside the battery, while a fuel cell produces the electricity from the fuel in an external fuel tank. This signifies that while a battery may run dead, a fuel cell will produce electricity as long as there is a supply of fuel.  

a is the correct answer