Final answer:
The gluteus medius (option a) is the correct site for an adult IM injection. It's chosen due to its depth and minimal risk of hitting nerves and blood vessels, unlike the gluteus maximus, which is larger but has more fat tissue and is near the sciatic nerve.
Step-by-step explanation:
The correct site for an adult intramuscular (IM) injection is the gluteus medius. This muscle is located in the gluteal region and is one of the muscles responsible for moving the femur, or thigh bone. It sits deep to the gluteus maximus and is found by imagining a quadrangle on the upper outer quadrant of the buttock; the injection should be administered in the upper outer quadrant of this area to avoid nerves and blood vessels.
While the gluteus maximus muscle is the largest, it is not typically the preferred site for IM injections due to the presence of significant fat tissue and proximity to the sciatic nerve. The gluteus medius offers a safer site that is less likely to result in complications. The gluteus minimus is too small to be a practical injection site, and the gastrocnemius muscle, which is located in the calf, is not an appropriate site for an IM injection in adults because it does not provide the necessary injection depth and volume accommodation and also has a higher risk for injury.