Answer: Rectangle
If you let
P = (0,0)
Q = (0,b)
R = (a,b)
S = (a,0)
you'll be able to find that
PQ is a vertical line with undefined slope
QR is a horizontal line with slope 0
so PQ and QR form a right angle as they are perpendicular lines
You'll find this with the other line pairings as well. We'll have four right angles