Final answer:
The correct reaction that uses water to break a molecule into smaller subunits is called hydrolysis, which adds water to split the bonds within polymers and form monomers.
Step-by-step explanation:
The reactions that use the equivalent of a water molecule to break a molecule into smaller subunits are known as hydrolysis reactions. During hydrolysis, a water molecule is inserted across the bond of a polymer, effectively breaking it into two components.
One part of the polymer gains a hydrogen atom (H+), and the other gains a hydroxyl group (OH-) from the split water molecule. This process is crucial for breaking down large biological molecules, like proteins, nucleic acids, and carbohydrates, into their smaller subunits or monomers.
In contrast, dehydration synthesis or condensation reactions are the reverse process where monomers combine to form polymers and release water as a byproduct.