The correct option is A.
A synthesis reaction is a type of reaction that occur between two or more elements which produce a single product. It is also called combination reaction.
For combination reaction, A + B = AB
For decomposition reaction, a single product is broken down into two or more components.
For single replacement reaction, a single free element will be substituted for one of the elements in a compound.
For double replacement reaction, two compounds exchange elements between each other.