**Core Concept**
The question tests the knowledge of non-depolarizing muscle relaxants (NDMRs) and their pharmacokinetic properties, specifically their duration of action. Doxacurium is a long-acting NDMR used in clinical practice.

**Why the Correct Answer is Right**
Doxacurium has the maximum duration of action among the given options due to its high potency and slow metabolism. It is metabolized by the liver and undergoes extensive enterohepatic recirculation, which contributes to its prolonged duration of action. Doxacurium has a long context-sensitive half-time, which means its effect persists even after the infusion is discontinued. This makes it suitable for procedures that require prolonged muscle relaxation.

**Why Each Wrong Option is Incorrect**
**Option A:** Atracurium has an intermediate duration of action, making it less suitable for procedures requiring prolonged muscle relaxation compared to Doxacurium.

**Option B:** Vecuronium has a moderate duration of action, which is shorter than Doxacurium's. It is primarily metabolized by the liver and excreted in the bile.

**Option C:** Rocuronium has a rapid onset and intermediate duration of action. It is metabolized by the liver and excreted in the bile, but its duration of action is shorter than Doxacurium's.

**Clinical Pearl / High-Yield Fact**
Doxacurium is a long-acting NDMR that should be used with caution in patients with renal or hepatic impairment, as its metabolism and excretion may be affected in these conditions.

**Correct Answer Line**
✓ Correct Answer: D. Doxacurium