Answer: D. synthesis reaction
Explanation:
A. Double displacement reaction
This reaction is defined as the chemical reaction in which exchange of ions takes place.
B. Single displacement reaction
This reaction is defined as the chemical reaction in which more reactive element displaces a less reactive element from its chemical reaction.
C. Decomposition reaction
This reaction is defined as the chemical reaction in which a single substance breaks down into two or more simpler substances.
D. Synthesis reaction
This reaction is defined as the chemical reaction in which substances combine in their elemental state to form a single compound.

Thus it is a synthesis reaction.