In a synthesis reaction, two or more elements are combined to form new compound. In decomposition reaction, the compound splits into two different elements. In single replacement reaction and double replacement atoms are replaced from one element to another element.
The given reaction is,
Here, represents the potassium chloride, is potassium, and represents the chlorine gas.
In this reaction, decomposes to K and Cl. Hence, it is an example for decomposition reaction.
Hence, the correct option is.