**Core Concept**
The physical half-life of a radioactive substance is a fundamental concept in nuclear medicine, referring to the time required for half of the radioactive atoms in a sample to decay, resulting in a decrease in radioactivity. In this context, the physical half-life of radioactive I-131 is a critical parameter in determining its suitability for diagnostic and therapeutic applications.

**Why the Correct Answer is Right**
Radioactive I-131 is a radioactive isotope of iodine with a physical half-life of approximately 8 days. This means that after 8 days, half of the I-131 atoms in a sample will have undergone radioactive decay, resulting in a decrease in radioactivity. The physical half-life of I-131 is a key factor in its use as a diagnostic and therapeutic agent in nuclear medicine, particularly in the treatment of thyroid disorders.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately reflect the physical half-life of radioactive I-131.
**Option B:** This option is incorrect because it is not a recognized value for the physical half-life of I-131.
**Option C:** This option is incorrect because it is not the correct value for the physical half-life of I-131.

**Clinical Pearl / High-Yield Fact**
The physical half-life of radioactive I-131 is a critical parameter in determining its suitability for diagnostic and therapeutic applications in nuclear medicine. It is essential to understand the physical half-life of I-131 to accurately predict its radioactivity and dosimetry in patients.

**Correct Answer: C. 8 days**