Which abbreviation denotes 'micrograms'?

The abbreviation that denotes 'micrograms' is indeed mcg. This abbreviation specifically represents one-millionth of a gram, which is a common measurement used in medicine, particularly when discussing dosages of medications or supplements.

Using 'mcg' helps healthcare professionals communicate dosages clearly and precisely, especially since micrograms are often used in contexts where small amounts of substances are significant, such as in hormone medications or vitamin supplements.

In contrast, the other choices represent different units: 'g' stands for grams, which is a larger unit of measure; 'ML' stands for milliliters, a unit of volume, and 'prn' is a Latin abbreviation meaning 'as needed,' typically referring to the administration of medication. Therefore, mcg is the correct choice and essential for understanding dosages in medical practices.