Decomposition:
A decomposition reaction occurs when one reactant breaks into two or more products. It can be represented like:
AB ----> A + B
This is not a decomposition reaction.
Single Replacement:
This one occurs when one element is replaced for another element in a compound. We can represent it like this:
AB + C ----> A + BC
This is not a single replacement reaction.
Synthesis:
It occurs when two different molecules or atoms form a different molecule. We can represent it like this:
A + B ----> AB
Our reaction is:
P₂O₃ + 3 H₂O -----> 2 H₃PO₃
We are combining both reactants to give the product.
Answer: Synthesis.