What is the special name for the chord RQ?

Draw a line segment $\overline{PQ}$ of length $8 \text{ cm}$.

At point P, use a compass and ruler to construct a perpendicular line to $\overline{PQ}$. This line will contain point R.

At point Q, use a compass and ruler to construct an angle of $30^\circ$ with $\overline{PQ}$. The ray forming this angle should intersect the perpendicular line from P. The intersection point is R.

Using a ruler, measure the length of the segment $\overline{RQ}$.
Theoretical calculation for verification:
In $\triangle PQR$, $\angle P = 90^\circ$ and $\angle Q = 30^\circ$.

Your measured value should be close to this.

a) (ii) Construct the perpendicular bisector (mediator) of RQ. Let it meet RQ at O.

Place the compass needle at R and open it to more than half the length of $\overline{RQ}$. Draw arcs above and below $\overline{RQ}$. Without changing the compass width, place the needle at Q and draw arcs above and below $\overline{RQ}$, intersecting the previous arcs. Draw a straight line through these two intersection points. This line is the perpendicular bisector of $\overline{RQ}$.

Mark the point where the perpendicular bisector intersects $\overline{RQ}$ as O.

b) With O as centre and radius OP, draw a circle. Measure $|OP|$.

Place the compass needle at O and open it to the length of $\overline{OP}$. Draw a circle. This circle should pass through points P, Q, and R.
Theoretical calculation for verification:
Since O is the midpoint of the hypotenuse $\overline{RQ}$ in the right-angled triangle PQR, O is the circumcenter, and thus $|OP| = |OQ| = |OR|$.

Your measured value should be close to this.

c) What is the special name for the chord RQ?

Since O is the center of the circle and O is the midpoint of $\overline{RQ}$, the chord $\overline{RQ}$ passes through the center of the circle.
The special name for the chord $\overline{RQ}$ is the diameter.