**Core Concept:**
The question is about drugs that can decrease the adverse effects of radiotherapy on the patient. Radiotherapy causes tissue damage by inducing cell death in rapidly dividing cells, but this also affects healthy cells leading to side effects. Chemotherapy drugs can interact with radiotherapy to reduce the toxicity.

**Why the Correct Answer is Right:**
The correct answer is **D.** Leucovorin (Folinic Acid). Leucovorin is a form of folic acid that is used as a component of the chemotherapy regimen called 5-fluorouracil (5-FU) for various cancers. When given together with 5-FU, leucovorin helps reduce the toxicity caused by radiotherapy. This is due to its ability to lower the toxicity of 5-FU by reversing its inhibition of thymidylate synthase, an enzyme involved in DNA synthesis and repair.

**Why Each Wrong Option is Incorrect:**
A. Option A (Methotrexate) is a folic acid antagonist that can increase the toxicity of radiotherapy when given together. This is because methotrexate inhibits the same enzyme (thymidylate synthase) that leucovorin helps reverse.

B. Option B (Cytosine arabinoside) is a nucleoside analogue used to treat certain cancers. While it can be used with radiotherapy, it is not specifically designed to reduce the toxicity of radiotherapy like leucovorin is.

C. Option C (Fluorouracil) is a nucleoside analogue used in chemotherapy to treat various cancers. While it can be used with radiotherapy, the focus is on leucovorin's ability to reduce the toxicity caused by radiotherapy.

**Clinical Pearl:**
Adjuvant therapy, which is the use of additional treatments after surgery to prevent cancer recurrence, is often used in conjunction with radiotherapy for various cancers, particularly in colorectal cancer treatment. Understanding the interactions between chemotherapy drugs and radiotherapy is essential for optimizing treatment outcomes while minimizing side effects.

**Correct Answer Explanation:**
Leucovorin (Folinic Acid) is the correct answer because it is a specific drug designed to reduce the toxicity caused by radiotherapy when used alongside 5-fluorouracil (5-FU). Leucovorin acts as a rescue agent for 5-FU toxicity, preventing the buildup of toxic intermediates and restoring the normal functioning of thymidylate synthase, an enzyme involved in DNA synthesis and repair.

**Why Each Wrong Option is Incorrect:**
Option A (Methotrexate) is a folic acid antagonist, meaning it competitively inhibits thymidylate synthase, a crucial enzyme in DNA synthesis and repair. By inhibiting this enzyme, methotrexate increases the toxicity of 5-FU and radiotherapy, unlike leucovorin.

Option B (Capecitabine) is an oral prodrug of 5-FU, which is converted to 5-FU after absorption. While this can be used with radiotherapy,