**Core Concept**
The question tests the student's knowledge of **antitumor agents** and their mechanisms of action, specifically those that target **nucleotide synthesis**. This concept is crucial in understanding how certain drugs exert their **cytotoxic effects** on cancer cells.

**Why the Correct Answer is Right**
The correct answer is related to the impairment of **de novo purine synthesis**, a pathway essential for DNA synthesis. This pathway involves several key enzymes, including **hypoxanthine-guanine phosphoribosyltransferase** and **adenine phosphoribosyltransferase**. An agent that impairs this pathway would effectively inhibit cancer cell proliferation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it refers to a different mechanism of action.
**Option B:** Similarly, this option is incorrect as it does not relate to the impairment of de novo purine synthesis.
**Option C:** This option is also incorrect as it is associated with a different antitumor mechanism.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **methotrexate**, an antifolate drug, works by inhibiting **dihydrofolate reductase**, an enzyme crucial for the synthesis of **tetrahydrofolate**, which is necessary for de novo purine synthesis. This makes it a prime example of a drug impairing de novo purine synthesis.

**Correct Answer:** D. Methotrexate.