Point C is in Quadrant IV.
Quadrant I is the top right, Quadrant II is the top left, Quadrant III is the bottom left, and Quadrant IV is the bottom right (it starts at the top right and goes clockwise around the graph).
The point (5, -3) goes 5 to the right and 3 down from the origin (0, 0), making it in the bottom right and therefore Quadrant IV.