This reaction can be classified as a combination reaction.
In this process, phosphorus pentoxide (P4O10) reacts with water (H2O) to form phosphoric acid (H3PO4). A combination reaction, also known as a synthesis reaction, occurs when two or more reactants combine to form a single product.
In this case, both P4O10 and water are combining to produce one product, H3PO4. Thus, it fits the criteria of a combination reaction. Other types of reactions listed, such as decomposition, combustion, and single displacement, do not apply here since no substances are breaking down, no energy is released in a flame-like reaction or one substance is displacing another in a compound.