Surface tension may be defined as:
A. The work done per unit area in increasing the surface area of a liquid under isothermal conditions
B. The work done per unit area in increasing the surface area of a liquid under adiabatic conditions
C. The work done per unit area in increasing the surface area of a liquid under isochoric conditions
D. Free surface energy per unit volume

Hint: Generally, when water is poured into any container it takes the shape of that container. But when the same water is poured on a greasy surface it takes the form of a spherical shape. This means that some forces other than gravity also work on the liquid.

Complete answer:
The behavior of liquids is controlled by gravity as well as other forces, which also depend on the surfaces the liquid is poured. The weight of the liquid is also responsible for its shape. If the weight of the liquid is negligible then the shape will be perfectly spherical. Therefore, liquid-free surfaces tend to achieve the minimum possible area.
This is explained based on a phenomenon called surface tension. The molecules inside the liquid experience forces from all sides and therefore net force becomes zero. But the molecule which is on the surface of a liquid experiences force only due to the bottom because of this it experiences some net force of attraction.
When the surface area of the liquid has increased the molecules from the interior come to the surface, by performing work against the downward force. This work is stored in the form of the potential energy of the surface. For any system to be in equilibrium, the energy must be minimum. So, to minimize this energy the number of molecules on the surface of liquid must be minimum. As a result, the liquid surface tries to take the minimum area.
Hence, the correct option is (A) the work done per unit area in increasing the surface area of a liquid under isothermal conditions.

Note:
We also come across a term called surface energy. When the surface area increases, cooling also happens. So, some heat from the surrounding flows to the surface of the liquid to keep its temperature constant. This additional energy is known as surface energy.
For a given volume the surface area of a sphere is minimum.