Which of the following units is commonly used to measure liquid ingredients?

Milliliters are commonly used to measure liquid ingredients because they are a metric unit specifically designed for volume measurement in liquids. This makes milliliters particularly suitable for measuring medications, solutions, and other liquid substances where precise volume control is critical. In contexts such as healthcare and cooking, using milliliters allows for accurate dosage and mixing, ensuring that the appropriate amount of a liquid ingredient is used.

The other units, while important in their own contexts, are primarily used for measuring mass rather than volume. Grams and milligrams are typically used for solid ingredients, and micrograms are used for very small quantities, which may not be practical for measuring liquids. In summary, milliliters provide a straightforward and widely accepted measure for liquids, which is why this unit is the correct choice in this question.