Question

hydrogen fuel cells are a promising source of energy. which of these is the \waste\ product produced by a hydrogen fuel cell? helium silver argon water

Explanation:

Hydrogen fuel cells work by combining hydrogen and oxygen. The chemical reaction between them produces electricity. The by - product of this reaction is water ($H_2O$). Helium ($He$), silver ($Ag$), and argon ($Ar$) are not involved in the basic hydrogen - oxygen fuel cell reaction.

Answer:

water