Join PR and ST so that they form a right angle with a vertex at the midpoint of ST.
1) In order to get it, we need to bisect the line segment ST. We can bisect that by intersecting ST with PR with a right angle right on the midpoint.
2) Thus, the answer is:
Join PR and ST so that they form a right angle with a vertex at the midpoint of ST.
3) Hence, the answer is "C"