Question

Which of the following is a secant?
A. AC
B. EF
C. GH
D. DE

Answer 1

It's B. EF 

The two points being intersected by the line is not on the circle however, the line still intersects two points and it passes through the circle. That clearly shows a secant line. 

AC is the diameter. (It's the line that divides the circle in half)
GH is a chord. (It is just a part of a secant. It connects two points on the circle.)
DE is a line segment.  (It's a part of tangent line since it lies outside the circle)

Answer 2

Let us identify each points.
AC is the diameter.
GH is a chord.
DE is a line segment. 

So the secant here is B. EF 

Secant is defined as:
The two points being intersected by the line is not on the circle however, the line still intersects two points and it passes through the circle.