**Core Concept**
Smokeless gun powder is a type of propellant used in firearms, designed to produce a rapid expansion of gases without generating significant smoke. This is achieved through the use of a specific nitrocellulose-based composition that combusts efficiently, producing a high-pressure gas.

**Why the Correct Answer is Right**
The correct answer, nitrocellulose, is a key component of smokeless gun powder. Nitrocellulose is a highly flammable, cellulose-based material that has been chemically modified to incorporate nitro groups, making it more stable and efficient as a propellant. When ignited, nitrocellulose undergoes a rapid decomposition reaction, releasing a large amount of gas and producing the high pressures necessary for propelling a projectile. The use of nitrocellulose in smokeless gun powder has largely replaced traditional black powder, which produced significant amounts of smoke and was less reliable.

**Why Each Wrong Option is Incorrect**
**Option A:** KMnO4 (Potassium permanganate) is a strong oxidizing agent used in various chemical reactions, but it is not a component of smokeless gun powder.
**Option B:** HCN (Hydrogen cyanide) is a highly toxic gas that can be produced in certain chemical reactions, but it is not a component of smokeless gun powder.
**Option D:** Sulphur is a component of traditional black powder, but it is not used in smokeless gun powder due to its tendency to produce significant amounts of smoke.

**Clinical Pearl / High-Yield Fact**
Smokeless gun powder is a critical component of forensic analysis in cases involving firearms. The presence and composition of smokeless gun powder can provide valuable evidence in determining the origin and trajectory of a projectile, as well as helping to identify potential suspects.

**✓ Correct Answer: C. Nitrocellulose**