A

According to Poiseuille’s law, blood flow (Q) is proportional to the fourth power of the vessel radius (Q ∝ r⁴). Increasing the radius by 50 % makes the new radius 1.5 r, so flow rises by (1.5)⁴ ≈ 5.06—about a five-fold increase [Pfitzner, 1976, PMID 779509].