The following reaction is a: \[2HgO \to 2Hg + {O_2}\]
A.Chemical combination reaction
B.Chemical displacement reaction
C.Chemical decomposition reaction
D.Physical change

Hint:To answer this question, you should chemical reaction and its types. A chemical reaction is in which the bonds are broken within reactant molecules, and new bonds are formed within product molecules to form a new substance.

Complete step by step answer:
1. Combination reaction: A combination reaction (also known as a synthesis reaction) is a reaction where two or more elements or compounds (reactants) combine to form a single compound (product).
Example: \[Cao + C{O_2} \to CaC{O_3}\]​, in this reaction calcium oxide is combined with carbon dioxide to form calcium carbonate.
2. Decomposition reaction: A Decomposition reaction is a reaction in which a single component breaks down into multiple products. Certain changes in energy in the environment have to be made like heat, light or electricity breaking bonds of the compound.
Consider the example of the decomposition of calcium carbonate \[CaC{O_{3}} \to CaO + C{O_2},\]in this reaction calcium carbonate is decomposed to calcium oxide and carbon dioxide.
3. Displacement reaction: A displacement reaction is the one wherein the atom or a set of atoms is displaced by another atom in a molecule.
\[Fe\left( s \right) + CuS{O_4}\left( {aq.} \right) \to FeS{O_{4}}\left( {aq} \right) + Cu\left( s \right),\] in this reaction, Fe displaces copper to form ferric sulphate. The reaction mentioned in the question: \[\;\;2HgO \to 2Hg + {O_2}\]. The reaction is reddish Mercuric oxide(II) solid is decomposed upon heating into silver liquid mercury and colourless oxygen gas. It is a chemical decomposition reaction as the mercuric oxide is broken down into mercury and oxygen.

Hence, the correct answer is option C.

Note:
A substance is said to undergo a physical change when only the physical properties of the reactant change while chemical change refers to a substance combined with another to form a new substance. Atoms or ions or molecules which react to form a new substance are called reactants; the new atoms or molecules formed are products. Chemical changes are irreversible except by further chemical reactions.