Answer: D. Arteries have a thicker wall relative to lumen diameter than veins. The wall-thickness-to-lumen-diameter ratios are P: 150/1000 = 0.15, Q: 30/300 = 0.10, R: 40/800 = 0.05 and S: 80/400 = 0.20. S has the greatest relative wall thickness.

A: chooses P by comparing absolute wall thickness alone; its wall is 150 µm, but its ratio is 0.15.
B: chooses Q by using the smallest lumen diameter alone rather than comparing wall thickness relative to diameter.
C: chooses R by reversing the criterion and selecting the smallest wall-to-lumen ratio.
Remember: identify an artery by its relatively thick wall, not by absolute wall thickness alone.