Question

Which one of the following is a decomposition reaction?

A. K2CO3 (s) → K2O (s) + CO2 (g)
B. 4Fe (s) + 3O2 (g) → 2Fe2O3 (s)
C. N2 (g) + 3H2 (g) → 2NH3 (g)
D. 2LiOH (s) + CO2 (g) → Li2CO3 (s) + H2O (l)

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.