## Core Concept
The half-life of a radioactive substance is the time required for half of the initial number of nuclei to decay. Radium is a radioactive metal with several isotopes, each having a different half-life.

## Why the Correct Answer is Right
Radium-226 (²²⁶Ra) is a well-known isotope of radium with a half-life of approximately 1600 years. This isotope is part of the uranium decay series and is itself a radioactive decay product. Its long half-life makes it useful for various applications but also poses long-term health risks due to its radioactivity.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because the value does not correspond with the known half-life of any prominent radium isotope.
- **Option B:** This option is incorrect as it suggests a much shorter half-life than that of radium-226, which might be more characteristic of other radioactive isotopes but not radium-226.
- **Option D:** This option is incorrect because it provides a half-life that is significantly longer than the accurately known half-life of radium-226.

## Clinical Pearl / High-Yield Fact
A key point to remember is that radium-226 has a half-life of 1600 years. This long half-life means that while radium-226 is hazardous due to its radioactivity, it decays very slowly. This property makes it useful in certain medical and industrial applications but also requires careful handling and disposal.

## Correct Answer Line
**Correct Answer: C. 1600 years.**