**Core Concept**
The question tests knowledge of **pharmacodynamics** and **pharmacokinetics** of muscle relaxants, specifically their duration of action. Muscle relaxants are crucial in **anesthesia** and **critical care** for facilitating tracheal intubation and providing muscle relaxation during surgery.

**Why the Correct Answer is Right**
The longest acting muscle relaxant is **Pancuronium**, due to its high **lipid solubility** and large **volume of distribution**, which prolongs its duration of action. Pancuronium acts by competitively inhibiting **nicotinic acetylcholine receptors** at the **neuromuscular junction**, leading to muscle relaxation.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not specified, and the correct answer is known to be Pancuronium.
**Option B:** Incorrect as it is not the longest acting, though it may have other desirable properties.
**Option C:** Incorrect, as while it may be used in anesthesia, it does not have the longest duration of action among muscle relaxants.
**Option D:** Similarly, incorrect as it does not match the known profile of the longest acting muscle relaxant.

**Clinical Pearl / High-Yield Fact**
Pancuronium's long duration of action makes it less frequently used in modern anesthesia practice, where shorter-acting agents are often preferred for their flexibility and safety profile. Remembering the duration of action of common muscle relaxants is crucial for **anesthesia management**.

**Correct Answer:** D. Pancuronium.