(7) The circle is usually denoted by the point at the center, so in this question the circle is labelled as P. The radius is the line that runs from the center to any point along the circumference (boundary) of the circle. Hence the radius here is either PR or PQ. While the diameter is the line that runs from one point along the circumference to another point along the circumference, and it MUST pass through the center, which is P. The circumference in this circle runs from point R to the other end of that long straight line (unfortunately the other end is not labelled in this diagram).
(8) The circle is labelled by the center H. The radius is labelled as HJ, HK and HL, while the diameter is labelled as KL.
(9) The circle is labelled by the center C. The radius is labelled as CD, CE and CF. The diameter is labelled as EF.