Question

Is ' surface tension', a scalar or a vector? why?

Answer 1

Surface tension is defined as the force acting per unit length of an imaginary line drawn on the surface of free liquid at right angle to the line . It is vector quantity because its direction toward imaginary line at right angle

Answer 2

$\huge \fbox \purple{Answer}$

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.