For types of reactions, we usually have:
Combination (also called synthesis): Where two elements or one compound and one element will bond and form a new compound as a product, the representation for this reaction is:
A + B -> AB
Decomposition: The opposite from Combination, now one compound will decompose and form 2 new elements or compounds as the product:
AB -> A + B
Single Replacement: One element will take the place of another element in already existing compound:
A + BC -> AB + C
Double Replacement: Now we have two compounds switching elements and forming two new compounds:
AB + CD -> AC + BD
In this case, we have an example of a Decomposition reaction, therefore letter D