**Core Concept**
The question tests knowledge of **chelating agents**, specifically those used to treat heavy metal poisoning. Chelation therapy involves using compounds that bind to heavy metals, facilitating their removal from the body.

**Why the Correct Answer is Right**
The correct answer is a chelating agent that can bind to **copper**, **mercury**, and **lead**, making it effective for treating poisoning by these metals. This agent is administered orally, which is crucial for managing patients with heavy metal exposure.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not the primary chelating agent for copper, mercury, and lead.
**Option B:** Incorrect as it is not typically used for oral administration in the context of heavy metal poisoning.
**Option C:** Incorrect because, although it may have some chelating properties, it is not the most appropriate choice for the specified metals.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that different chelating agents have affinity for different heavy metals, and the choice of agent depends on the specific metal involved in the poisoning.

**Correct Answer:** D. Succimer