**Core Concept**
The question is testing the knowledge of mucolytic agents, specifically their mechanism of action, which involves breaking disulfide bonds in mucus to reduce viscosity. Mucolytics can be classified based on their mechanism of action, and the presence of a free sulphydryl group is a key characteristic of certain agents.

**Why the Correct Answer is Right**
The correct answer is Acetylcysteine (N-acetylcysteine, NAC). Acetylcysteine contains a free sulphydryl group (-SH) that acts as a reducing agent to break disulfide bonds in mucus, thereby reducing its viscosity. This mechanism of action is unique to acetylcysteine among the mucolytic agents, making it the correct answer. The presence of a free sulphydryl group is essential for the mucolytic activity of acetylcysteine.

**Why Each Wrong Option is Incorrect**
**Option A:** **No information available** (Note: The question lacked information for options A, B, and C)
**Option B:** **No information available**
**Option D:** **No information available**
Please note that the original question lacked information for options A, B, and C.

**Clinical Pearl / High-Yield Fact**
Acetylcysteine has multiple clinical uses beyond mucolysis, including its role as an antidote for acetaminophen (paracetamol) overdose and as a mucoprotective agent to reduce the risk of ventilator-associated pneumonia.

**Correct Answer:** A. Acetylcysteine.