The length of line segment PR of the geometric system of similar triangles is equal to 6 units (Correct choice: B).
How to determine the missing length in a system of proportional triangles
Herein we find a geometric system formed by two similar triangles, that is, two triangles that have congruent angles and proportional sides. The length of line segment PR by proportion formula:

If we know that AB = 20, PQ = 5 and AC = 24, then line segment PR of the geometric system is:

PR = 24 / 4
PR = 6
The system of similar triangles has the following missing length: 6 units.