### Core Concept
Radiosensitizers are agents that increase the sensitivity of tumor cells to radiation therapy, enhancing the effectiveness of cancer treatment. They work by modifying cellular processes that repair DNA damage caused by radiation, thereby making cells more susceptible to radiation-induced cell death.

### Why the Correct Answer is Right
Misonidazole is a classic radiosensitizer that acts by inhibiting the enzyme aldehyde dehydrogenase, which is involved in the repair of radiation-induced DNA damage. By blocking this enzyme, misonidazole increases the accumulation of radiation-induced DNA damage, making tumor cells more vulnerable to radiation therapy.

### Why Each Wrong Option is Incorrect
**Option A:** Hydroxyurea is a radioprotector, not a radiosensitizer. It works by inhibiting the enzyme ribonucleotide reductase, which is involved in DNA synthesis, thereby protecting normal cells from radiation-induced damage.
**Option B:** Cisplatin is a DNA-damaging chemotherapeutic agent, but it is not typically classified as a radiosensitizer. While it can enhance the effectiveness of radiation therapy, its primary mechanism is not through radiosensitization.
**Option C:** Misonidazole is the correct answer, as previously explained.

### Clinical Pearl / High-Yield Fact
Radiosensitizers can be broadly categorized into two types: chemical radiosensitizers (e.g., misonidazole) and hypoxic radiosensitizers (e.g., tirapazamine). The latter class targets hypoxic tumor cells, which are more resistant to radiation therapy.

### Correct Answer: C. Misonidazole