HgO + Cl2 → HgCl + O2

A. Double Replacement
B. Single Replacement
C. Combustion
D. Decomposition
E. Synthesis

B. Single Replacement

Explanation:

Single replacement:

It is the reaction in which one elements replace the other element in compound.

AB + C → AC + B

Chemical equation:

2HgO + Cl₂ → HgCl + O₂

This is the single replacement reaction. In this reaction chlorine replace the oxygen from mercury oxide and form mercury chloride.

Other options are incorrect because,

Decomposition reaction:

It is the reaction in which one reactant is break down into two or more product.

AB → A + B

Synthesis reaction:

It is the reaction in which two or more simple substance react to give one or more complex product.

A + B → AB

Double replacement:

It is the reaction in which two compound exchange their ions and form new compounds.

AB + CD → AC +BD