**Core Concept**
Trachoma is a leading cause of preventable blindness worldwide, caused by chronic infection with Chlamydia trachomatis. Mass treatment is a strategy to control and eliminate the disease in endemic areas.

**Why the Correct Answer is Right**
The World Health Organization (WHO) recommends mass treatment for trachoma when the prevalence of severe or moderate trachoma in children under 10 years reaches 10% or higher. This threshold is based on the fact that trachoma is highly infectious and spreads rapidly in areas with high prevalence. Mass treatment aims to reduce the reservoir of infection and prevent further transmission. Azithromycin, an antibiotic, is the preferred treatment for trachoma.

**Why Each Wrong Option is Incorrect**
* **Option A:** 5% is too low, as mass treatment is not recommended for this level of prevalence.
* **Option B:** 8% is below the threshold for mass treatment, as per WHO guidelines.
* **Option D:** 15% is above the threshold, but this is not the correct answer as it is not the specific threshold mentioned in the question.

**Clinical Pearl / High-Yield Fact**
Remember the WHO guidelines for mass treatment of trachoma: treat when the prevalence of severe or moderate trachoma in children under 10 years reaches 10% or higher.

**Correct Answer:** C. 10%