Answer: Option (C) is the correct answer.
Step-by-step explanation:
When two different compounds combine to result in the formation of a new compound then this type of reaction is known as synthesis reaction.
On the other hand, when a compound decomposes to result in the formation of different compounds then it is known as decomposition reaction.
Therefore, the chemical equation for the given reaction is as follows.


Thus, we can see that carbonic acid dissociates to give water and carbon dioxide. Both water and carbon dioxide are compound.
Hence, we can conclude that it is a decomposition reaction; carbonic acid breaks down into two simpler compounds.