**Core Concept**
Chlorhexidine is a broad-spectrum antimicrobial agent used for skin disinfection, particularly for hand antisepsis. Its effectiveness is concentration-dependent, with optimal activity observed at specific concentrations.

**Why the Correct Answer is Right**
Chlorhexidine works by binding to the bacterial cell membrane, disrupting its function and ultimately leading to cell lysis. The optimal concentration of chlorhexidine gluconate for hand antisepsis is 2% in an aqueous solution, which is effective against a wide range of microorganisms, including Gram-positive and Gram-negative bacteria, viruses, and fungi. This concentration is critical for ensuring the antimicrobial activity of chlorhexidine.

**Why Each Wrong Option is Incorrect**
**Option A:** 0.5% - This concentration is too low to be effective against most microorganisms, making it unsuitable for hand disinfection.
**Option B:** 4% - While higher concentrations may be more effective against certain organisms, 4% chlorhexidine can cause skin irritation and is not recommended for hand antisepsis.
**Option C:** 1% - This concentration is not sufficient to achieve optimal antimicrobial activity, making it a less effective option for hand disinfection.

**Clinical Pearl / High-Yield Fact**
When using chlorhexidine for hand disinfection, it is essential to rinse hands thoroughly with water after application to prevent skin irritation and ensure effective antimicrobial activity.

**Correct Answer: C. 0.5% - 2%