The correct answer is A: Splitting of water into hydrogen and oxygen.
A decomposition reaction is a type of chemical reaction in which a compound breaks down into simpler substances. In option A, water (H₂O) is decomposing into its constituent elements, hydrogen (H₂) and oxygen (O₂). This reaction can be represented as:
2H₂O → 2H₂ + O₂
Option B describes a combustion reaction, where carbon is burning in oxygen to produce carbon dioxide. Option C represents a combination reaction, as calcium oxide combines with water to form calcium hydroxide. Option D is an example of a redox reaction, where oxygen combines with carbon monoxide to produce carbon dioxide.