In circle O, which segment is a chord?

A chord of a circle is a straight line segment whose endpoints both lie on the circle.

More generally, a chord is a line segment joining two points on any curve, for instance an ellipse.

A chord that passes through a circle's center point is the circle's diameter.

Every diameter is a chord, but not every chord is a diameter.

The first choice, OH is not a chord, it is the radius of the circle. 

SD is also not a chord, it is a tangent line to the circle. 

OR is too, not a chord, it is also a radius. 

RC is a chord, since a diameter is a chord. 

:)