For 1.
Always , every diameter of a circle is also a chord because a chord of a circle is any line segment joining two points on the circle's circumference
For 2.
Never, the radius only intersects one point of the circle.
For 3.
Never, A-line that is tangent to a circle intersects the circle in only one point
For 4.
Sometimes, only when the two radii are in the same line they form the diameter of a circle
For 5.
Always, because a secant will touch two points of a circle, and contain a chord.
For 6.
Sometimes, that is not necessary that a chord touches the center, but can be possible.