1. The problem asks why milliliters (mL) are more appropriate than fluid ounces (fl oz) for scientific measurement in a protocol. 2. Important fact: 1 fl oz ≈ 29.57 mL. 3. Milliliters are part of the metric system, which is used internationally in science because it standardizes units and makes data more easily shared and understood across countries. 4. Using milliliters reduces errors and improves reproducibility because all scientists can directly compare measurements without conversion mistakes. 5. Fluid ounces are not part of the metric system and are less commonly used in science, so relying on milliliters aligns with scientific standards. 6. The other options (smaller unit, expense, and labeling) are either incorrect or less relevant than the standardization benefit. 7. Therefore, the most correct answer is: "Milliliters are part of the metric system, which is the international standard for science, reducing errors and improving reproducibility."