**Core Concept**
Intravenous (IV) insulin administration requires a specific preparation to ensure proper solubilization and delivery. The correct preparation is crucial to prevent precipitation, aggregation, or other adverse effects that could compromise patient safety.

**Why the Correct Answer is Right**
The best preparation of insulin for IV injection is **Regular Insulin (Actrapid or Humulin R) diluted in 50 mL of 0.9% sodium chloride solution**. This is because Regular Insulin is a short-acting insulin that is readily soluble in water, and diluting it in 0.9% sodium chloride solution helps to prevent precipitation and ensures a stable solution. This preparation is commonly used in critical care settings for the management of severe hyperglycemia or diabetic ketoacidosis.

**Why Each Wrong Option is Incorrect**
**Option A:** **NPH Insulin (Humulin N or Lente)** is an intermediate-acting insulin that is not suitable for IV administration due to its slower onset and peak action, which may not be compatible with the rapid delivery of IV insulin.
**Option B:** **Lente Insulin** is also an intermediate-acting insulin that is not suitable for IV administration due to its variable onset and peak action, which may not be predictable in a critical care setting.
**Option C:** **Ultralente Insulin** is a long-acting insulin that is not suitable for IV administration due to its prolonged action, which may not be compatible with the rapid delivery of IV insulin.

**Clinical Pearl / High-Yield Fact**
When preparing insulin for IV administration, it is essential to use a syringe with a 25-gauge or smaller needle to minimize the risk of insulin precipitation or aggregation.

**Correct Answer:** C. Lente Insulin.