Answer:
Synthesis
Explanation:
Two reactants are becoming one product, so it is a synthesis reaction.
Decomposition is the opposite, so when one molecule becomes simpler atoms.
Example: 2NaCl -> Cl2 + Na
Combustion is reaction with a product of water and carbon dioxide.
Example: CH4 + 2O2 -> CO2 + 2H2O
Single displacement is when one atom takes the place of the other.
Example: K + NaCl -> KCl +Na (where K took the place of Na)
Double displacement is like single displacement but it happens twice. Think of it like the atoms exchanging partners.
Example: HCl +NaH -> NaCl + HOH