The correct answer is C.
A is wrong answer, because you can put the rule under the different angles to the side QR and as the correct angle is unknown you cannot make this step.
B is a wrong choice, because you'll only measure the radius of the circle and still you don't know which point on the second circle is on the side RX.
D is a wrong choice, because you can put a set of segments of this length from the point R, but which of them will form the needed angle is still unknown.
C is a right choice, because you measure the length of chord ST and then put the point on the second circle, which will form the equal chord.
