Final answer:
The posterior surface of the upper arm is the appropriate site for a subcutaneous injection among the options provided, due to its adequate fat layer for medication absorption.
Step-by-step explanation:
When administering a subcutaneous injection, choosing the right site is crucial to ensure proper absorption and minimize discomfort. Among the options provided, the posterior surface of the upper arm is an appropriate site for a subcutaneous injection. This site is commonly used for injections because it has a sufficient layer of fat beneath the skin, which is ideal for the absorption of subcutaneously administered medications. The anterior to the sternum, dorsal surface of the hand, and medial aspect of the lower leg are not commonly recommended sites for subcutaneous injections due to the presence of underlying structures and thinner layers of skin and fat, which could increase the risk of pain or injury during the injection.