Achemical reaction has two elements as reactants. which type of reaction might occur?
asynthesis
b)decomposition
c)single replacement
d)double replacement

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.

Sorry i think the answer is b