An international unit of enzyme activity is defined as the quantity of enzyme that can:

The correct choice describes an international unit of enzyme activity as the amount of enzyme that can convert 1 micromole of substrate to product per minute. This definition highlights the functionality of enzymes and emphasizes their activity based on substrate conversion, which is a common measure of enzyme effectiveness in biochemical assays.

This unit is a standardized measure used across various publications and studies, ensuring that enzyme activity can be accurately compared and understood within the scientific community. It reflects the enzyme's efficiency and catalytic capability, which is crucial in both clinical and research settings to assess enzyme kinetics and substrate availability.

The other options do not align with the established definition of enzyme activity units. For example, breaking down 1 milligram of product per hour does not provide a clear measurement of conversion rate in terms of substrate to product, which is vital for determining enzyme activity. Similarly, producing 1 liter of substrate per day or activating 1 unit of substrate within 10 minutes lack standard biochemical context and measurable precision in enzyme activity, making them unsuitable definitions.