What is it? Mg + O2 + Mgo
1. Decomposition
2.Double Displacement 3.Synthesis
4. Single Displacement

The correct reaction is as follows:

2Mg + O2 → 2MgO

3. Synthesis

Explanation:

Chemical reactions are of different types depending on how they occur. One of these types is called SYNTHESIS OR COMBINATION REACTION. A synthesis reaction is that reaction in which two atoms of two elements are chemically combined to form a single molecule or compound. That is;

A + B → AB.

This is the case in this question where atoms of Magnesium (Mg) and oxygen (O) reacted to form a single compound of Magnesium oxide (MgO). Hence, it is said to be a SYNTHESIS REACTION.