**Core Concept**
The question is testing the knowledge of long-acting muscle relaxants, which are used in the management of patients with neuromuscular diseases or those undergoing surgery. These medications act by competing with acetylcholine for receptors at the neuromuscular junction, thereby reducing muscle contraction force.

**Why the Correct Answer is Right**
The correct answer is succinylcholine (Option C: Succinylcholine), a depolarizing muscle relaxant that acts on the nicotinic receptors at the neuromuscular junction. Succinylcholine is a long-acting muscle relaxant because it is metabolized slowly by plasma cholinesterase, an enzyme that breaks down acetylcholine. As a result, succinylcholine can produce prolonged neuromuscular blockade, lasting up to 60 minutes.

**Why Each Wrong Option is Incorrect**
**Option A:** Vecuronium is an intermediate-acting non-depolarizing muscle relaxant, not a long-acting one. Vecuronium is metabolized by the liver and excreted in the bile and urine.
**Option B:** Rocuronium is also an intermediate-acting non-depolarizing muscle relaxant, similar to vecuronium. While rocuronium has a rapid onset of action, it is not a long-acting muscle relaxant.
**Option D:** Atracurium is an intermediate-acting non-depolarizing muscle relaxant that undergoes Hofmann elimination, a spontaneous degradation process that is independent of liver or kidney function.

**Clinical Pearl / High-Yield Fact**
Succinylcholine can cause an increase in serum potassium levels due to the depolarization of muscle cells, which can be a concern in patients with burns, trauma, or muscle damage.

**Correct Answer: C. Succinylcholine**