**Core Concept**
Radioprotection involves the use of agents to mitigate the harmful effects of ionizing radiation on living tissues. Radioprotectors are substances that can reduce the damage caused by radiation, thereby enhancing the tolerance of tissues to radiation.

**Why the Correct Answer is Right**
Amifostine is a radioprotector that works by scavenging free radicals and inhibiting the depletion of glutathione, a crucial antioxidant in cells. It also inhibits the enzyme topoisomerase, which is involved in DNA repair. By reducing the amount of free radicals and preserving glutathione levels, amifostine helps to protect normal tissues from radiation damage. This is particularly useful in reducing the incidence of radiation-induced xerostomia (dry mouth) and nephrotoxicity in patients undergoing radiation therapy.

**Why Each Wrong Option is Incorrect**
**Option A:** Hydroxyurea is an antineoplastic agent that works by inhibiting DNA synthesis, but it is not a radioprotector.
**Option B:** Cyclophosphamide is a chemotherapeutic agent that is used to treat various types of cancer, but it does not have radioprotective properties.
**Option C:** This option is missing.

**Clinical Pearl / High-Yield Fact**
One of the key benefits of radioprotectors like amifostine is their ability to reduce the incidence of radiation-induced side effects, thereby improving the quality of life for patients undergoing radiation therapy.

**Correct Answer:** A. Amifostine.