## Core Concept
The half-life of a radioactive substance is the time it takes for half of the radioactive atoms in a sample to decay. Iodine-131 is a radioactive isotope of iodine commonly used in nuclear medicine for diagnostic and therapeutic purposes. Understanding its half-life is crucial for calculating the duration of its therapeutic effects and the period during which it poses a radiation hazard.

## Why the Correct Answer is Right
The half-life of Iodine-131 is precisely 8 days. This means that every 8 days, the radioactivity of a sample of Iodine-131 decreases by half due to nuclear decay. This property makes Iodine-131 particularly useful for certain medical treatments, such as ablating the thyroid gland in thyroid cancer patients or treating hyperthyroidism. Its relatively short half-life allows for therapeutic effects without prolonged radiation exposure.

## Why Each Wrong Option is Incorrect
- **Option A:** 2 days - This is too short for the half-life of Iodine-131. A half-life of 2 days would mean the substance decays much faster than Iodine-131 actually does.
- **Option B:** 6 days - While close, this is still shorter than the actual half-life of Iodine-131. A half-life of 6 days would underestimate the duration of radioactivity of Iodine-131.
- **Option D:** 10 days - This is slightly longer than the actual half-life. A half-life of 10 days would overestimate the duration for which Iodine-131 remains radioactive.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that Iodine-131 is used not only for its therapeutic effects but also for diagnostic imaging, particularly in assessing thyroid function and cancer. Knowing its half-life helps in planning treatment and minimizing radiation exposure to patients and healthcare workers.

## Correct Answer Line
**Correct Answer: C. 8 days.**