Answer is: B. Decomposition.
Decomposition is reaction where one substance is broken down into two or more simpler substances.
Electrolysis of water is the decomposition reaction, because from one molecule (water) two molecules (hydrogen and oxygen) are produced.
Water is separeted into two molecules:
Reaction of reduction at cathode: 2H⁺(aq) + 2e⁻ → H₂(g).
Reaction of oxidation at anode: 2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e⁻.