Hello!
Before I can answer your question we need to know the different types of chemical reactions that can occur:
- Single Replacement
- Double Replacement
- Synthesis
- Decomposition
- Combustion
Combustion is an answer that can usually be ruled out unless oxidation is occuring in the chemical equation (hyrdrogen occuring with oxygen to react)
Synthesis reaction, also known as a Combination reaction, is when the reactants combine to form a single product.
Decompostion is when a single reactant is deconstructed to form multiple products, or the opposite of Synthesis.
With these definitions alone, since they are opposite types of reactions, the answer would be Synthesis and Decomposition reactions !
Hope this helps and if you need anymore clarification feel free to ask.