## Core Concept
Technetium-99m (Tc-99m) is a metastable nuclear isomer that is widely used in nuclear medicine for diagnostic imaging. Its half-life is a critical property that determines its utility for various medical applications. The half-life of a radioactive substance is the time required for half of the initial number of nuclei to decay.

## Why the Correct Answer is Right
The correct half-life of Tc-99m is approximately 6 hours. This relatively short half-life makes Tc-99m ideal for medical imaging because it allows for the administration of a radioactive dose that decays quickly, minimizing the patient's exposure to radiation. The half-life of 6 hours also enables the performance of imaging studies within a reasonable timeframe after administration.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because the half-life mentioned does not match the known half-life of Tc-99m.
- **Option B:** This option suggests a much longer half-life than that of Tc-99m, which does not align with its properties.
- **Option D:** This option suggests a half-life that is too short compared to the actual half-life of Tc-99m.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that the short half-life of Tc-99m, combined with its suitable gamma radiation energy, makes it an excellent choice for a variety of diagnostic imaging procedures, including bone scans, cardiac stress tests, and certain types of tumor imaging. Its properties help in achieving high-quality images while keeping radiation exposure to patients as low as reasonably achievable.

## Correct Answer: C. 6 hours