**Core Concept**
The question is testing the knowledge of non-depolarizing muscle relaxants, specifically their duration of action. Non-depolarizing muscle relaxants are a class of drugs that compete with acetylcholine for receptors at the neuromuscular junction, leading to muscle relaxation.

**Why the Correct Answer is Right**
The shortest acting non-depolarizing muscle relaxant is **Mivacurium**. It is a derivative of atracurium and has a rapid onset and short duration of action due to its ester linkage, which is hydrolyzed by plasma cholinesterase. This results in a brief duration of action, typically lasting around 15-20 minutes. Mivacurium is often used in clinical settings where rapid reversal of muscle relaxation is desired.

**Why Each Wrong Option is Incorrect**
**Option A:** **Vecuronium** is a non-depolarizing muscle relaxant with a moderate duration of action, typically lasting around 30-40 minutes. It is not the shortest acting non-depolarizing muscle relaxant.
**Option B:** **Rocuronium** has a rapid onset of action but a longer duration of action compared to Mivacurium, typically lasting around 30-60 minutes. It is not the shortest acting non-depolarizing muscle relaxant.
**Option C:** **Pancuronium** is a long-acting non-depolarizing muscle relaxant, typically lasting around 60-120 minutes. It is not the shortest acting non-depolarizing muscle relaxant.

**Clinical Pearl / High-Yield Fact**
Mivacurium's short duration of action makes it a useful choice in patients with renal or hepatic impairment, where longer-acting muscle relaxants may accumulate to toxic levels.

**Correct Answer: D. Mivacurium**