**Core Concept**
In managing diabetic ketoacidosis (DKA) in an elderly patient, the primary goal is to restore euglycemia while avoiding the overcorrection of hyperglycemia and the risk of hypokalemia. Insulin therapy is crucial in this process.

**Why the Correct Answer is Right**
The drug of choice in DKA is insulin, specifically intravenous regular insulin, which helps to decrease glucose levels, suppress ketone production, and promote the uptake of glucose by cells. The administration of intravenous insulin allows for precise control of glucose levels and is particularly useful in elderly patients who may have decreased renal function and be at risk for hyperkalemia. The use of subcutaneous insulin in this context is not ideal due to its slower onset of action and variable absorption.

**Why Each Wrong Option is Incorrect**
**Option A:** Oral hypoglycemic agents such as metformin or sulfonylureas are not the first-line treatment for DKA, as they may worsen the condition by stimulating insulin release and increasing glucose levels.
**Option B:** Glucagon is actually contraindicated in DKA as it can exacerbate the condition by increasing glucose levels and ketone production.
**Option C:** Thiazide diuretics can worsen dehydration and electrolyte imbalances in DKA, making them an inappropriate choice for treatment.

**Clinical Pearl / High-Yield Fact**
In elderly patients with DKA, it's essential to monitor potassium levels closely and consider administering potassium supplements to prevent hypokalemia. A common mistake in DKA management is to administer insulin without adequate potassium replacement.

**Correct Answer:** D. Insulin.