Scale of Biological Structures Steps:

• Alveoli diameters range from 200-300 μm, requiring micrometers (μm) for measurement.
• White blood cell diameters are 10-20 μm, also fitting μm scale.
• Cell widths, often referring to membrane thickness or fine structures, are around 5-10 nm, suiting nanometers (nm). Why D is correct:
• Matches the hierarchical scales: μm for larger cellular structures like alveoli and WBCs, nm for sub-cellular widths per standard biological sizing conventions. Why the others are wrong:
• A: mm is too large for alveoli (millimeter scale suits organs, not sacs).
• B: nm omitted; ignores nanoscale for cell widths.
• C: mm for WBCs is excessively large (WBCs are microscopic).

Final answer: D