Final answer:
Solubility defines a liquid's ability to mix with water. It measures how much a substance can dissolve in water, which relies on the substance's interactions with water molecules.
Step-by-step explanation:
The correct answer to the student's question about a liquid's ability to mix with water is solubility. Solubility is a measure of the amount of a substance that can dissolve in a given amount of solvent at a specified temperature and pressure. For liquids mixing with water, the key factor is the solute's capability to interact with water molecules through intermolecular forces like hydrogen bonds and dipole interactions.
Other options such as surface tension, water reactivity, and instability do not directly describe a liquid's ability to mix with water. Surface tension is the energy or force at the surface of a liquid that results in a minimized surface area, due to the attractive forces between the molecules on the surface of the liquid. It can be affected by solutes like surfactants, which can lower water's surface tension, but it does not describe the process of mixing or dissolving.