**Core Concept**
The administration of insulin is a crucial aspect of managing diabetes mellitus, and understanding the most common route of administration is vital for effective glucose control. Insulin can be administered via various routes, including subcutaneous, intramuscular, intravenous, and intraperitoneal injection. However, the subcutaneous route is the most common due to its ease of use and effectiveness.

**Why the Correct Answer is Right**
The subcutaneous route involves injecting insulin into the fatty tissue just beneath the skin, where it is absorbed slowly into the bloodstream. This route allows for a controlled release of insulin, which helps to regulate blood glucose levels. The subcutaneous tissue has a rich blood supply, which facilitates the absorption of insulin into the bloodstream. Additionally, the subcutaneous route allows for the use of insulin pens, syringes, and pumps, making it a convenient and accessible option for patients.

**Why Each Wrong Option is Incorrect**
**Option A:** Intramuscular injection is not the most common route of administration due to the risk of muscle atrophy, pain, and bleeding.
**Option B:** Intravenous injection is typically reserved for acute management of hyperglycemia or diabetic ketoacidosis, as it allows for rapid insulin delivery but can lead to hypoglycemia.
**Option C:** Intraperitoneal injection involves injecting insulin into the abdominal cavity and is not a common route of administration due to the risk of infection and peritonitis.

**Clinical Pearl / High-Yield Fact**
The subcutaneous route of insulin administration is most effective when injections are given at a 90-degree angle to the skin, using a needle length of 8-10 mm, and injecting at a rate of 1-2 seconds per injection.

**Correct Answer: C. Subcutaneous injection.**