## Core Concept
The physical half-life of a radioactive substance is the time it takes for half of the radioactive atoms in a sample to decay. This is a fundamental concept in nuclear physics and medicine, particularly relevant for radioactive isotopes used in diagnosis and treatment, such as iodine-131.

## Why the Correct Answer is Right
Iodine-131, also known as I-131, has a well-established physical half-life that is crucial for its medical applications, including the treatment of thyroid cancer and hyperthyroidism. The correct half-life of iodine-131 is approximately 8 days. This duration allows for the therapeutic effect of the radiation while also considering the safety and handling of the radioactive material.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests a half-life of 2.4 minutes, which is far too short for iodine-131 and more characteristic of certain other radioactive isotopes used in medicine or research.
- **Option B:** This option proposes a half-life of 2.4 hours, still much shorter than the actual half-life of iodine-131 and not consistent with its medical use.
- **Option D:** This option states a half-life of 240 days, which is significantly longer than the actual half-life of iodine-131 and not reflective of its physical properties.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that the biological half-life of iodine-131 (the time it takes for the body to eliminate half of the iodine) is different from its physical half-life. For iodine-131, the effective half-life (a combination of physical and biological half-lives) in the human body is about 5-7 days, but its physical half-life remains approximately 8 days.

## Correct Answer: C. 8 days.