**Core Concept**
The recommended concentration of free chlorine is crucial for effective disinfection of water, particularly in healthcare settings, to prevent the spread of waterborne pathogens. Free chlorine is a disinfectant that kills microorganisms by denaturing proteins and disrupting cellular membranes.

**Why the Correct Answer is Right**
The correct concentration of free chlorine is 5.25 mg/L, which is sufficient to inactivate viruses, bacteria, and other microorganisms. This concentration is achieved by adding chlorine tablets or solutions to water, allowing for adequate contact time to ensure effective disinfection. The World Health Organization (WHO) recommends this concentration for water treatment to prevent waterborne diseases.

**Why Each Wrong Option is Incorrect**
**Option A:** 1 mg/L is too low to effectively kill microorganisms and may not provide adequate disinfection.
**Option B:** 10 mg/L is excessively high and can be corrosive to water distribution systems and damage equipment.
**Option C:** 2.5 mg/L, while closer to the correct answer, is still insufficient to ensure complete disinfection of water.

**Clinical Pearl / High-Yield Fact**
Remember, the correct concentration of free chlorine is 5.25 mg/L for effective water disinfection, as this concentration has been proven to inactivate a wide range of microorganisms, including viruses, bacteria, and protozoa.

**Correct Answer: D. 5.25 mg/L**