Answer:
A -- only
Step-by-step explanation:
A segment bisector intersects a segment at its midpoint.
S is the midpoint of segment PQ, so ST is a segment bisector. (It intersects PQ at its midpoint, S.)
No other midpoints are shown in this drawing.
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A perpendicular bisector is perpendicular to the segment it bisects. None are shown in this drawing.
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An angle bisector divides an angle into two congruent parts. None are shown in this drawing.
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The vertex of a right angle is often marked with a small square. Only point P is the vertex of a right angle in this drawing. (We cannot assume OPQR is a square.)
