Pretty self explanatory title. But I just read in a book that the surface tension remains same even if the surface area is increased. What could be a possible explanation for that?
The surface tension is a consequence of the interfacial energy. If you consider some area of the liquid surface $A$ then there is an interfacial energy proportional to the area:
$$ E = kA $$
for some constant $k$ that is dependent on the forces between the atoms/molecules in the liquid. If we increase the area of the liquid surface we increase its energy, and that means we have to do work on it. If we do work we must have been exerting a force on the boundaries of the surface, and the force per unit length of the boundary is what we call the surface tension.
The interfacial energy is a consequence of the atomic/molecular interactions in the liquid so the interfacial energy per unit area is a constant and not dependent on the total area. That means the surface tension is also dependent only on the interactions within the liquid and not on the total area.