**Core Concept**
The half-life of a radioactive substance like Iodine-131 (I-131) is the time it takes for the radioactivity to decrease by half due to nuclear decay. This concept is crucial in nuclear medicine, particularly in the diagnosis and treatment of thyroid disorders.

**Why the Correct Answer is Right**
I-131 has a half-life of approximately 8 days (or 192 hours). This means that every 8 days, the amount of radioactive iodine in the body will decrease by half. This property makes I-131 useful for diagnostic imaging and therapeutic purposes, as it can be used to destroy thyroid tissue while minimizing radiation exposure to other parts of the body. The decay of I-131 is a result of beta and gamma radiation emission.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as the half-life of I-131 is not 3 days. This would imply a much faster decay rate than observed.
**Option B:** Incorrect, as the half-life of I-131 is not 24 hours. This would imply a much shorter duration of radioactivity than actual.
**Option C:** Incorrect, as the half-life of I-131 is not 30 days. This would imply a longer duration of radioactivity than actual.

**Clinical Pearl / High-Yield Fact**
The half-life of I-131 is a critical factor in determining the optimal dosing and treatment duration for thyroid cancer and hyperthyroidism. A thorough understanding of this concept is essential for nuclear medicine practitioners.

**Correct Answer: A. 8 days**