Where is a subcutaneous injection administered?

A subcutaneous injection is administered under the skin, which is the correct answer. This method involves delivering medication into the subcutaneous tissue layer, just beneath the dermis and epidermis of the skin. The subcutaneous layer contains fat and connective tissue, which allows for slow absorption of the medication into the bloodstream.

This route is commonly used for medications like insulin and certain vaccines, as it provides a sustained release of the drug over time. In contrast, options such as administering into the muscle, bloodstream, or veins describe different routes of injection—intramuscular, intravenous, and intravascular respectively—which serve specific purposes and have different absorption rates.