Which property correctly distinguishes liquids from solids?

The correct option is "Liquids have fixed volume but no fixed shape".

To understand why this is the correct distinguishing property, let us analyze the physical properties of solids and liquids based on the kinetic molecular theory of matter:

1. Arrangement and Behavior of Particles:
In solids, particles are tightly packed in a regular, fixed pattern. The intermolecular forces of attraction are very strong, which keeps the particles in fixed positions. As a result, solids have both a fixed shape and a fixed volume.
In liquids, the particles are still close together, but they are not locked in a rigid arrangement. The intermolecular forces are strong enough to keep the particles close (giving them a fixed volume), but weak enough to allow them to slide past one another. This ability to flow allows liquids to take the shape of whatever container they are placed in, meaning they do not have a fixed shape.

2. Evaluating the Options:
- "Liquids cannot diffuse into other substances": This is incorrect. Liquids can diffuse into other liquids (for example, ink diffusing in water).
- "Liquids are completely incompressible in nature": This is incorrect. While liquids are highly resistant to compression compared to gases, they are not completely incompressible; under extreme pressure, their volume can change slightly.
- "Liquids have fixed shape but no fixed volume": This is incorrect. It is the exact opposite of how liquids behave. Liquids flow to take the shape of their container (no fixed shape) but maintain a constant volume.
- "Liquids have fixed volume but no fixed shape": This is correct. A given quantity of liquid (e.g., 100 mL of water) will occupy the same volume whether it is in a beaker, a flask, or a bowl, but its shape will change to match the container.