Which reaction below represents a balanced, decomposition chemical reaction?

a)
2mgo → mg + o2

b)
2al + 3br2 → 2albr3

c)
2kclo3 → 2kcl + 3o2

d)
c5h12 + 8o2 → 5co2 + 6h2o

We can first rule out choices B and D because in decomposition reactions, one reactant is decomposed or broken up to form two or more products. Choices B and D are incorrect because they both have two reactants. 

Then we need to determine which choice, A or C, is correctly balanced. Choice A is incorrectly balanced because there are 2 moles of magnesium on the reactants side but only one mole of magnesium on the products side. Therefore, C is the correct choice because it is correctly balanced.

Hope this helps.