Question

Which one of the following is a decomposition reaction?

Answer 1

You would know a decomposition reaction occurred if the reactants separated. For example from AB → A+B.

Now if you look at your options only 1 works out for that equation. Letter A. 
From the compound K2CO3 it split up to K2O +CO2

It cannot be letter B because synthesis/combination occurred. The same goes for letter C. Letter D, single displacement occurred. 

Again, the answer is A.