**Core Concept:** Chemo-radiation therapy is a combination of chemotherapy and radiotherapy, used to treat various types of cancer. Radiotherapy causes damage to healthy cells, leading to side effects and reduced quality of life. The question asks about a drug that can reduce the toxicity caused by radiotherapy.

**Why the Correct Answer is Right:** The correct answer, **Option C: Dexamethasone**, is a corticosteroid used to reduce inflammation and swelling related to radiotherapy. Dexamethasone works by suppressing the immune system and reducing the release of inflammatory mediators. This helps to minimize damage to healthy cells, thereby reducing the side effects of radiotherapy.

**Why Each Wrong Option is Incorrect:**

A. **Option A: 5-Fluorouracil (5-FU):** 5-FU is an antimetabolite used in chemotherapy, targeting rapidly dividing cells. While it is effective against cancer cells, it does not specifically target the toxicity caused by radiotherapy.

B. **Option B: Leucovorin:** Leucovorin is a folate analogue used in combination with 5-FU in chemotherapy. It is used to reduce toxicity due to 5-FU but does not specifically target the toxicity caused by radiotherapy.

C. **Option C: Dexamethasone:** Dexamethasone reduces inflammation and swelling caused by radiotherapy, thereby reducing the toxicity to healthy cells, making it the correct answer.

D. **Option D: Cisplatin:** Cisplatin is a platinum-based chemotherapeutic agent used in cancer treatment. It is not specifically targeted at reducing the toxicity caused by radiotherapy.

**Clinical Pearl:** In clinical practice, dexamethasone is commonly used along with radiotherapy to minimize side effects and improve patient comfort. However, it is essential to balance the benefits of using corticosteroids with the potential downsides, such as increasing infection risk and altering immune response.