Which of the following is a secant? a. ac b. ef c. gh d. de

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)

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