(B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
Both statements are factually correct, but R describes what Bowman's capsule is — it does not explain why the filtration unit is a capillary cluster.
The Assertion is directly from the textbook: "the basic filtration unit in the kidneys, like in the lungs, is a cluster of very thin-walled blood capillaries." The Reason is also correct: "Each capillary cluster in the kidney is associated with the cup-shaped end of a coiled tube called Bowman's capsule." However, R merely names/describes the structure associated with the capillary cluster; it does not explain the assertion about capillaries being the filtration unit. Hence option (B) is correct.