A chord is a line segment that has two points located somewhere around the circle, and when connected, separate the circle into two pieces. For your information, the diameter of the circle is also a chord, one that perfectly separates the circle into two identical pieces.
The only line segment that satisfies this is the segment QR, or A. Points Q and R are located on the circle, and are connected. RS isn’t a chord because point S isn’t located on the circle.
