Scale conversion using image proportions

Steps:

• Scale bar image length Z corresponds to actual 15 μm.
• Ratio Y/Z determines how many scale bar units fit in image diameter Y.
• Actual diameter equals this ratio times actual scale length: (Y/Z) × 15.
• Option C matches this calculation directly.

Why C is correct:

• It uses the proportion actual/Y = 15/Z, solved as actual = (Y/Z) × 15, standard for scale bars.

Why the others are wrong:

• A: Places 15 before ratio, misrepresenting proportion setup (should be ratio first for unit count).
• B: Writes Y × (15/Z) but skips explicit ratio Y/Z, risking miscalculation of scale units.
• D: Gives only scale factor 15/Z, omitting multiplication by image diameter Y.

Final answer: C