**Core Concept**
The question requires knowledge of chelating agents, which are compounds that form stable complexes with metal ions, including heavy metals like mercury. In the context of mercury poisoning, a chelating agent is used to bind and remove the toxic metal from the body.

**Why the Correct Answer is Right**
The correct answer is Dimercaprol (also known as BAL), which is a sulfur-containing chelating agent that forms a stable complex with mercury ions. Dimercaprol works by binding to the mercury ion through its sulfur atoms, forming a stable complex that can be excreted from the body. This process helps to reduce the toxicity of mercury and facilitate its elimination.

**Why Each Wrong Option is Incorrect**
**Option A:** Penicillamine is a chelating agent used primarily to treat heavy metal poisoning, but it is more commonly used for copper and gold poisoning, not mercury.
**Option B:** Succimer is a chelating agent used to treat lead poisoning, not mercury poisoning.
**Option C:** Deferoxamine is a chelating agent used to treat iron poisoning, not mercury poisoning.

**Clinical Pearl / High-Yield Fact**
When treating heavy metal poisoning, it's essential to choose the correct chelating agent based on the specific metal involved, as each agent has a different affinity for different metals.

**Correct Answer: D. Dimercaprol**