Question

reasons for the surface tension is scalar or a vector

Answer 1

Answer is surface tension will be along positive and negative y direction, since it's perpendicular to the line and acts on both sides. So you see it's the line that determines the direction of the force, not the surface tension. So it's a scalar quantity.

Answer 2

surface tension is a scalar quantity because surface tension =work donesurface area, where both work done and area are scalars.