Is pressure a vector, scalar or tensor quantity. Why?

is a scalar quantity. Not a vector quantity.(Mind you vector and scalar are rank-1 and rank-0 tensor, so your question about being a tensor is a meaningless one, as tensor is both vector and scalar and what not, so yes it is a tensor.)

And as to why it is a scalar: It's because you always take the force in the direction perpendicular to area, what it basically say is that you take the component of the force in the direction of area (remember the direction of area is perpendicular to the surface!) and divide it by the area. So you see even though ##P=F/A##

F is not a vector here, it's the component of F in the area's direction, which is not a vector(which is is usually also not a scalar by the way, but here the component's direction is fixed and invariant under coordinate rotation, so here it's a scalar), so pressure comes out as a scalar.

The above answer is for isotropic case, if it's not isotropic then pressure is a rank-2 tensor.