Answer:
<em>C: ST</em>
Step-by-step explanation:
<u>Perpendicular Lines</u>
The four points R(-5, 3), S(-5, 4), T(-3, 4), and U(-3, 1) are shown in the image attached below.
The vertical line TU is also shown. The only possible line perpendicular to TU is ST because it's a horizontal line.
Horizontal lines have a slope of 0. Let's verify the slope between points S(-5,4) and T(-3,4):

Once verified, the answer is C: ST