46. Surface tension may be defined as

(a)  The work done per unit area in increasing the surface area of a liquid under isothermal condition

(b)  The work done per unit area in increasing the surface area of a liquid under adiabatic condition

(c)   The work done per unit area in increasing the surface area of a liquid under both isothermal and adiabatic conditions

(d)   Free surface energy per unit volume

Ans. a