Electrolysis of water is

Correct answer: (b) Decomposition reaction

Explanation

In the electrolysis of water an electric current breaks water into its elements:

2H₂O(l) → 2H₂(g) + O₂(g)

One compound gives two simpler substances, so it is a decomposition reaction (an electrolytic decomposition). Nothing is displaced and nothing combines, so it is neither a displacement nor a combination reaction.

Some answer keys give (d) More than one of the above. Chemically, the electrolysis of water is only a decomposition reaction.