**Core Concept**
The World Health Organization (WHO) guidelines for trachoma control recommend mass treatment with azithromycin in areas with a high prevalence of trachoma. The key indicator for mass treatment is the prevalence of follicular trachomatous inflammation (TF) in children aged 1-9 years.

**Why the Correct Answer is Right**
Mass treatment with azithromycin is indicated when the prevalence of TF in children aged 1-9 years exceeds 10%. This is because trachoma is a major public health problem in areas with high prevalence, and mass treatment can help control the disease and prevent its complications. Azithromycin targets the bacteria Chlamydia trachomatis, which causes trachoma. The high prevalence of TF in children indicates a high risk of transmission and complications, making mass treatment necessary.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the WHO guidelines do not recommend mass treatment at a prevalence below 10%.
* **Option B:** This option is incorrect because the prevalence threshold for mass treatment is not 20%, but rather 10%.
* **Option C:** This option is incorrect because there is no specific prevalence threshold of 30% mentioned in the WHO guidelines for mass treatment with azithromycin.

**Clinical Pearl / High-Yield Fact**
The WHO recommends mass treatment with azithromycin every 2-3 years in areas with a high prevalence of trachoma, to control the disease and prevent its complications.

**Correct Answer:** B. 10%