Final answer:
Casey used a compass centered at point P with a radius equal to PQ to draw the arcs that defined points M and N. Therefore, the correct option is: d. She used a compass centered at P with a radius set equal to PQ.
Step-by-step explanation:
To inscribe an equilateral triangle within a circle using a compass and a straightedge, Casey would use a compass centered at P with a radius set equal to PQ (the diameter of the circle). Therefore, the correct option is: d. She used a compass centered at P with a radius set equal to PQ. This method ensures that the arcs drawn from P intersect the circle at points M and N such that the resulting triangle triangle MNP is equilateral.