2mgo à 2mg + o2 is an example of a chemical reaction.
a. synthesis
b. combustion
c. replacement
d. decomposition

2MgO → 2Mg + O2 is an example of a decomposition chemical reaction. The answer is letter D. It is not a replacement reaction because no other compound is involved. It is not a combustion reaction because no oxygen is involved.

D. decomposition

Explanation:

Synthesis reaction occurs when two elements or compounds react to originate a new one:

A + B ⇒ C  

Combustion occurs when a fuel reacts with oxygen to form CO₂ and water:

A + O₂ ⇒ CO₂ + H₂O

Replacement reaction can be simple or double. Simple occurs when a simple element replaces others in a compound:

A + BC ⇒ B + AC

Double occurs when two compounds replace elements between them:

AB + CD ⇒ AC + BD

Decomposition reaction occurs when a compound originates two or more compounds

A ⇒ B + C

So, the reaction given is a decomposition chemical reaction.