What type of reaction is shown below?

Synthesis

Explanation:

Chemical equation:

3S + 2Fe → Fe₂S₃

The solid iron react with solid sulfur and form iron(III) sulfide. The reaction type is synthesis because,

Synthesis reaction:

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

A + B  → AB

Other options are incorrect because,

In double displacement  two compound exchange their ions and form new compounds.

AB + CD → AC +BD

In decomposition one reactant is break down into two or more product.

AB → A + B

In single replacement one elements replace the other element in compound.

AB + C → AC + B