## Core Concept
The question refers to a significant medical discovery in the 20th century. This period saw numerous groundbreaking findings, but one stands out as particularly transformative.

## Why the Correct Answer is Right
The discovery of **insulin** is often hailed as the 'medical discovery of the 20th century'. Insulin, a hormone produced by the pancreas, regulates blood glucose levels. Its discovery by Frederick Banting and Charles Best in 1921 revolutionized the treatment of diabetes, transforming it from a fatal disease to a manageable condition.

## Why Each Wrong Option is Incorrect
- **Option A:** Although not specified, any other option not related to insulin would be incorrect because they do not represent the discovery hailed as the 'medical discovery of the 20th century'.
- **Option B:** Similarly, without specifics, any option not directly linked to the discovery of insulin or its impact would be incorrect.
- **Option C:** This would also be incorrect for the same reason; it does not pertain to the discovery of insulin.

## Clinical Pearl / High-Yield Fact
The discovery of insulin is a landmark in medical history. **Frederick Banting and Charles Best** isolated insulin in 1921, and it has since become a lifesaving treatment for diabetes mellitus. This discovery not only saved countless lives but also paved the way for modern endocrinology and the development of treatments for various hormonal imbalances.

## Correct Answer Line
**Correct Answer: D. Insulin.**