When electricity is passed through water, what kind of chemical reaction occurs?

Correct — A. Passing electricity through water splits it:
2H₂O → 2H₂ + O₂

One compound breaks down into two simpler substances, and that is the definition of a decomposition reaction.

Because the energy driving it is electrical, it is called electrolytic decomposition, or electrolysis.

Hydrogen collects at the cathode and oxygen at the anode in a 2:1 volume ratio. The reaction is endothermic — cut the current and it stops.

Why the others are wrong

• (b)
  Double displacement — Double displacement exchanges ions between two compounds, as barium chloride and sodium sulphate do. Here only one compound is present.
• (c)
  Combination — Combination is the reverse of what happens — two or more substances joining into a single product, as hydrogen does when it burns in oxygen to form water.
• (d)
  Displacement — Displacement needs a more reactive element to push a less reactive one out of its compound, like iron in copper sulphate solution. No such element is added to the water.

Decomposition reactions break a single compound into two or more products, classified by the form of energy that supplies the push.

Thermal decomposition uses heat: limestone → lime + carbon dioxide.

Electrolytic decomposition uses current: water → hydrogen + oxygen.

Photolytic decomposition uses light: silver chloride → silver + chlorine.

All three absorb energy, and that shared thread is usually the real point of the question.

Key facts

• Electrolysis of water: 2H₂O → 2H₂ + O₂, with hydrogen at the cathode and oxygen at the anode in a 2:1 volume ratio.
• Decomposition driven by heat, by electricity and by light is called thermal, electrolytic and photolytic decomposition respectively.
• Decomposition reactions absorb energy, which is why every one of them needs heat, light or current supplied.
• Combination reactions run the other way and usually release energy.

Common traps

• Calling it displacement because two gases 'come out' — nothing is displaced, the compound simply breaks apart.
• Missing that the 2:1 hydrogen-to-oxygen ratio follows from the formula of water, not from the voltage applied.

No directly related past PYQ was found.