**Core Concept**
The question is testing the student's knowledge of the isotopes used in thyroid scanning. Thyroid scanning is a diagnostic imaging technique used to visualize the thyroid gland and assess its function. The correct isotope for thyroid scanning should have properties that allow it to accumulate in the thyroid gland and be detectable by imaging techniques.

**Why the Correct Answer is Right**
The correct answer is **Technetium-99m**. Technetium-99m is a metastable nuclear isomer that decays to technetium-99 with the emission of gamma rays. It is commonly used in nuclear medicine for diagnostic imaging, including thyroid scanning. Technetium-99m has a half-life of 6 hours and emits gamma rays at an energy level of 140 keV, making it ideal for imaging. It is taken up by the thyroid gland due to the presence of iodine transport mechanisms, allowing for the visualization of the thyroid gland.

**Why Each Wrong Option is Incorrect**
**Option A:** Iodine-129 is a radioactive isotope of iodine with a half-life of 15.7 million years. It is not commonly used for thyroid scanning due to its long half-life and low energy gamma rays.
**Option B:** Iodine-131 is a radioactive isotope of iodine used in the treatment of thyroid disorders, but it is not typically used for diagnostic thyroid scanning due to its high energy gamma rays and potential for radiation exposure.
**Option D:** Selenium is a non-radioactive element that is not used in thyroid scanning.

**Clinical Pearl / High-Yield Fact**
Remember that Technetium-99m is the most commonly used isotope for thyroid scanning due to its favorable properties and widespread availability.

**Correct Answer:** C. Technetium-99.