**Core Concept**
The mass treatment of trachoma, also known as the SAFE strategy, is a public health approach to control and prevent the disease. The SAFE strategy includes Surgery for trichiasis, Antibiotics to treat infection, Facial cleanliness, and Environmental improvement. The mass treatment of trachoma is a key component of this strategy, aimed at reducing the prevalence of the disease in a community.

**Why the Correct Answer is Right**
Mass treatment of trachoma is recommended when the prevalence of trachoma in a community is at least 10%. This is based on the World Health Organization's (WHO) guidelines for the prevention of blindness due to trachoma. The decision to implement mass treatment is made when the prevalence of trachoma in children aged 1-9 years reaches 10%, indicating a high burden of the disease in the community. The antibiotics used in mass treatment target the causative agent of trachoma, Chlamydia trachomatis, and help to reduce the transmission of the disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** There is no specific prevalence threshold below which mass treatment is not recommended. However, mass treatment is typically targeted to communities with a high prevalence of trachoma.
* **Option B:** There is no evidence to suggest that mass treatment is recommended at a prevalence of 20%. The WHO guidelines recommend mass treatment at a prevalence of 10%.
* **Option C:** This option is incorrect because mass treatment is not recommended at a prevalence of 5%. While treatment may be targeted to specific individuals or groups, mass treatment is generally recommended at a higher prevalence threshold.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the SAFE strategy is not a one-time intervention but a long-term approach to controlling and preventing trachoma. The strategy requires sustained efforts to improve facial cleanliness, environmental conditions, and access to antibiotics and surgery.

**Correct Answer: C. 10%**