Which of the following is an example of a surface property?

Surface tension of a solution is a property that specifically relates to the interface between the liquid and the air (or another fluid) above it. It arises due to the cohesive forces between liquid molecules at the surface, which result in a minimized surface area. This phenomenon leads to behaviors such as droplets forming and insects being able to walk on water.

The other properties mentioned do not specifically pertain to the surface of a liquid. For instance, density is a bulk property that reflects how much mass of the solution is contained in a given volume, and it doesn’t fundamentally change based on surface interactions. Viscosity measures a liquid's resistance to flow, which is also a characteristic not tied to the surface of a liquid but rather to its internal structure and interactions. Solubility of a solute refers to how well it can dissolve in a solvent, which deals with the interaction at the molecular level within the bulk solution rather than at the surface.

Thus, surface tension is distinctly a surface property, making it the correct choice among the options provided.