## **Core Concept**
Phenol and its derivatives are widely used as disinfectants due to their broad-spectrum antimicrobial activity. The effectiveness of phenol depends on its concentration, with higher concentrations generally providing greater antimicrobial efficacy. For the disinfection of feces, a specific concentration of phenol is recommended to ensure effective microbial killing without posing significant risks to humans.

## **Why the Correct Answer is Right**
The correct answer, **C. 5%**, is the appropriate concentration of phenol for the disinfection of feces. A 5% phenol solution is commonly recommended for this purpose because it provides a good balance between antimicrobial efficacy and safety. This concentration is effective against a wide range of bacteria, viruses, and fungi that may be present in fecal material, thereby reducing the risk of disease transmission.

## **Why Each Wrong Option is Incorrect**
- **Option A: 1%** - This concentration is likely too low to effectively disinfect fecal material, as it may not provide sufficient antimicrobial activity against all potential pathogens.
- **Option B: 2%** - Although closer to the effective range, 2% might still be insufficient for reliable disinfection of feces, especially against more resistant organisms.
- **Option D: 10%** - While a higher concentration of phenol would be more effective against a broader range of pathogens, it is not typically necessary for routine disinfection of feces and could pose greater risks to human health and the environment.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that phenol and phenolic compounds are used in various concentrations for different disinfection purposes. For instance, a 5% phenol solution is specifically noted for its use in disinfecting fecal material, making it a valuable piece of information for public health and infection control practices.

## **Correct Answer: C. 5%.**