**Core Concept**
Radioisotopes are used in teletherapy to deliver high-energy radiation for cancer treatment. Teletherapy involves external beam radiation therapy where the radiation source is outside the body. The choice of isotope depends on its energy level, half-life, and availability.

**Why the Correct Answer is Right**
Cobalt-60 (Co-60) is a commonly used isotope in teletherapy due to its optimal energy level of 1.25 MeV and half-life of 5.26 years. This allows for efficient delivery of radiation to the tumor site with minimal damage to surrounding healthy tissues. Co-60 also has a relatively long half-life, making it easier to maintain and replace. The gamma radiation emitted by Co-60 is absorbed by the tumor, causing DNA damage and ultimately leading to cell death.

**Why Each Wrong Option is Incorrect**
**Option A:** Iodine-123 (I-123) is primarily used in nuclear medicine for imaging purposes, such as thyroid function tests and tumor localization. It has a shorter half-life and lower energy level, making it unsuitable for teletherapy.

**Option B:** Caesium-137 (Cs-137) is also used in radiation sources, but its energy level (662 keV) is lower than Co-60, and its half-life (30.2 years) is longer, which may lead to increased radiation exposure and handling challenges.

**Option D:** Technetium-99 (Tc-99) is a metastable nuclear isomer that decays to Tc-99m, which is commonly used in nuclear medicine for imaging and diagnostic purposes. It is not suitable for teletherapy due to its low energy level and short half-life.

**Clinical Pearl / High-Yield Fact**
When choosing a radioisotope for teletherapy, it's essential to consider the energy level, half-life, and availability to ensure efficient and safe radiation delivery to the tumor site.

**✓ Correct Answer: C. Co-60**