**Core Concept**
The question requires knowledge of neuromuscular blocking agents (NMBAs) and their pharmacological properties, particularly their onset of action. NMBAs are a class of drugs used to facilitate tracheal intubation and provide muscle relaxation during surgery.

**Why the Correct Answer is Right**
The correct answer is **Succinylcholine** (SCh). SCh is a depolarizing NMBA that acts by activating nicotinic acetylcholine receptors at the neuromuscular junction, leading to muscle contraction and eventually paralysis. Its onset of action is the shortest among NMBAs, typically occurring within 30-60 seconds. This rapid onset is due to its ability to rapidly diffuse across the NMJ, where it binds to and activates receptors.

**Why Each Wrong Option is Incorrect**
**Option A:** Atracurium - Atracurium is an intermediate-acting non-depolarizing NMBA, which has a slower onset of action compared to SCh.
**Option B:** Rocuronium - Rocuronium is another non-depolarizing NMBA, but it has a slightly slower onset of action compared to SCh, typically occurring within 60-90 seconds.
**Option C:** Pancuronium - Pancuronium is a long-acting non-depolarizing NMBA, which has a much slower onset of action compared to SCh.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the rapid onset of SCh can be a double-edged sword, as it can also increase the risk of hyperkalemia, particularly in patients with underlying muscle damage or electrolyte imbalances.

**Correct Answer:** D. Succinylcholine