Filtration of waste products in humans happens in the kidneys. There is a large number of filtration units present inside the kidneys to help them in doing this job. These filtration units are called:

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1. The functional filtration unit of the kidney is the nephron: each nephron filters blood and forms urine.

2. A nephron consists of a renal corpuscle (glomerulus + Bowman's capsule) and a renal tubule; Bowman's capsule alone (option a) is only a part of the nephron, not the whole filtration unit.

3. Capillaries (option c) are blood vessels and alveoli (option d) are air sacs in the lungs, so neither are kidney filtration units.

4. For reference, each human kidney contains on the order of $10^{6}$ nephrons, underscoring that nephrons are the multiple filtration units in kidneys.

Therefore the correct choice is (2) nephrons → option b.

FINAL_ANSWER: b

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