When the surface of a liquid resists any external force due to the cohesive nature of its molecules it is known as surface tension.
These cohesive forces between the molecules are shared with all the neighboring molecules. The molecules at the surface does not have any neighboring molecules thus, they are attracted by the molecules on and below the surface.
This reduces the surface area and increases the surface tension of the liquid. More tightly the molecules are attracted, more will be the surface tension.
Out of the given options, mercury has the highest surface tension.
