Which of the following medications is commonly administered via subcutaneous injection?

Insulin is commonly administered via subcutaneous injection because this route allows for the medication to be absorbed gradually into the bloodstream, which is particularly crucial for managing blood sugar levels in patients with diabetes. Subcutaneous injections involve depositing the medication into the fatty tissue beneath the skin, which provides a slower absorption rate compared to intravenous or intramuscular routes. This slower absorption is beneficial for insulin, as it helps maintain stable blood glucose levels over time.

In contrast, other medications listed are typically administered via different routes. For instance, penicillin may be given intramuscularly or intravenously depending on the formulation and clinical situation, while the pneumococcal vaccine is generally administered intramuscularly as well. Intravenous fluids are delivered directly into the bloodstream through intravenous administration to ensure rapid effect, which is not the intention with subcutaneous injections. Therefore, insulin's requirement for a slower and sustained release makes it the correct choice for subcutaneous administration.