We are given a circle with center O.
We can see that we have a radius OB of given circle.
OC and OE are another two radii.
EC is a diameter of the circle.
Diameter is the segment that cut the circle at two points and passes through center.
AB is a tangent of the given circle.
A tangent is a segment that cut a circle at one point only.
But the chord is GD.
A chord of a circle is the segment that cut a circle at two points.
Therefore, a chord in circle O is GD.