Molecules of liquid are attracted to what property that causes them to stick together on the surface?

Surface tension is the property that causes molecules of a liquid to stick together at the surface. It arises from the cohesive forces between liquid molecules, which are stronger at the interface compared to those in the bulk of the liquid. This cohesive force leads to the formation of a "skin" on the surface of the liquid that minimizes surface area and is responsible for phenomena like droplets forming, insects walking on water, and the ability of some objects to float despite being denser than the liquid.

While viscosity pertains to the internal friction of a fluid and resistance to flow, and gravity relates to the weight and attraction of objects towards the center of the Earth, it is the cohesive nature of liquid molecules that specifically leads to surface tension. Cohesion itself describes the attraction between like molecules which contributes to surface tension but does not define the outward property that is observed at the liquid's surface. Thus, surface tension is the correct answer as it describes this phenomenon specifically.