Answer: C. 6
Explanation:
In the given picture , we have two triangles ΔABC and ΔPQR in which
∠B=∠Q=75° and ∠C=∠R=55°
By AA-Similarity postulate ΔABC ≈ ΔPQR.
We know that the corresponding sides of two similar triangles are proportional.
Therefore , we have

Hence, the length of PR = 6 units.