**Core Concept**
Mercury is a heavy metal that exists in various forms, each with different toxicities and risks to human health. The toxicity of mercury is influenced by its chemical properties, solubility, and ability to cross biological barriers.

**Why the Correct Answer is Right**
The most toxic form of mercury is methylmercury (CH3Hg+), a potent neurotoxin that accumulates in the food chain, particularly in fish and shellfish. Methylmercury is formed through the methylation of inorganic mercury by microorganisms in aquatic ecosystems. It crosses the blood-brain barrier and binds to sulfhydryl groups in proteins and enzymes, disrupting cellular function and leading to neurodegeneration.

**Why Each Wrong Option is Incorrect**
**Option A:** Inorganic mercury (Hg2+), while toxic, is less potent than methylmercury due to its lower bioavailability and solubility in water. It tends to accumulate in tissues and is excreted more slowly.
**Option B:** Elemental mercury (Hg0), also known as quicksilver, is less toxic than methylmercury due to its low solubility in water and limited bioavailability. However, it can vaporize and cause respiratory toxicity.
**Option C:** Mercuric chloride (HgCl2) is a highly toxic compound, but its toxicity is primarily due to its corrosive and irritant properties, rather than its neurotoxic effects.

**Clinical Pearl / High-Yield Fact**
Methylmercury exposure is particularly concerning for pregnant women and children, as it can cause permanent neurological damage and developmental delays.

**Correct Answer: D. Methylmercury.