Which is a secant?
CT
MH
CD

The secant in the given figure is:
CD
We know that in geometry a secant is a line that intersect the circle or any curve in atleast two distinct points.
Here we have a curve as a circle.
Hence, the secant is: CD
( Also MH is not a secant since it does not intersect the curve at any point.
Also CT is not a secant because it is not a line, it is a line segment although it intersect the curve at two distinct points )