**Core Concept**
The question is testing the understanding of insulin therapy in diabetes mellitus, specifically the choice of insulin for treatment. Insulin therapy is a cornerstone in managing diabetes mellitus, and the selection of the appropriate insulin is crucial for effective glycemic control.

**Why the Correct Answer is Right**
The correct answer is Regular Insulin, which is a short-acting insulin analog. Regular insulin is the insulin of choice for the treatment of diabetes mellitus because it provides a rapid onset of action and a relatively short duration of action, making it ideal for managing postprandial glucose spikes. Regular insulin works by binding to insulin receptors on the surface of cells, activating the insulin signaling pathway, and promoting glucose uptake into cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** NPH Insulin is a medium-acting insulin analog, which is not the insulin of choice for treating diabetes mellitus. NPH insulin has a slower onset of action and a longer duration of action compared to Regular Insulin.
* **Option B:** Ultralente Insulin is a long-acting insulin analog, which is not suitable for managing postprandial glucose spikes. Ultralente insulin has a slow onset of action and a prolonged duration of action, making it unsuitable for acute glucose control.
* **Option C:** Lente Insulin is a medium-acting insulin analog, which is not the insulin of choice for treating diabetes mellitus. Lente insulin has a slower onset of action and a longer duration of action compared to Regular Insulin.

**Clinical Pearl / High-Yield Fact**
Regular Insulin is a short-acting insulin analog that is ideal for managing postprandial glucose spikes. It is often administered before meals to prevent hyperglycemia.

**Correct Answer:** A. Regular Insulin