Explanation:

Surface tension is a physical property of liquids that arises due to the cohesive forces between the molecules at the surface of the liquid. It is defined as the force per unit length acting perpendicular to any line drawn on the surface of the liquid.

a) Acts in the plane of the interface normal to any line in the surface:
Surface tension acts in the plane of the interface which is normal to any line in the surface. This means that the force acting on the surface is always perpendicular to any line drawn on the surface.

b) Also known as capillarity:
Surface tension is also known as capillarity because it is responsible for the phenomenon of capillary action, which is the ability of a liquid to flow in narrow spaces against the force of gravity.

c) A function of the curvature of the interface:
Surface tension is a function of the curvature of the interface. The more curved the surface, the higher the surface tension. This is because the cohesive forces between the molecules are stronger at a curved surface than at a flat surface.

d) Decreases with fall in temperature:
Surface tension decreases with a fall in temperature. This is because at lower temperatures, the molecules have less thermal energy and are less mobile, which weakens the cohesive forces between them.

e) Has no units:
Surface tension is measured in units of force per unit length, such as newtons per meter (N/m) or dynes per centimeter (dyn/cm). However, it is important to note that surface tension itself is a property and does not have any units.

Conclusion:
Surface tension is an important property of liquids that arises due to the cohesive forces between the molecules at the surface of the liquid. It acts perpendicular to any line drawn on the surface and is a function of the curvature of the interface. It decreases with a fall in temperature and is measured in units of force per unit length.