**Core Concept**
Visual acuity is a critical parameter in assessing vision loss, and referral guidelines are essential for early detection and treatment of eye diseases. The World Health Organization (WHO) and other health organizations have established guidelines for referral to Primary Health Care (PHC) based on visual acuity.

**Why the Correct Answer is Right**
The correct answer is based on the WHO's guidelines for referral to PHC, which recommend referral for a patient with visual acuity of less than 6/18 in the better eye. This threshold is used because it indicates significant vision loss that may require further evaluation and treatment by an eye specialist.

* The WHO guidelines are based on the need to detect and treat eye diseases early to prevent permanent vision loss.
* A visual acuity of 6/18 or worse indicates a significant reduction in visual function that may affect daily activities and quality of life.
* Early referral to PHC allows for timely assessment and treatment of eye diseases, which can prevent further vision loss and improve outcomes.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not align with the WHO's guidelines for referral to PHC.
* **Option B:** This option is incorrect because it is too stringent, and patients with visual acuity between 6/18 and 6/60 may still require referral to PHC for further evaluation and treatment.
* **Option D:** This option is incorrect because it is too broad, and patients with visual acuity between 6/60 and 6/120 may not necessarily require referral to PHC.

**Clinical Pearl / High-Yield Fact**
Remember that visual acuity is a critical parameter in assessing vision loss, and referral guidelines are essential for early detection and treatment of eye diseases. The WHO's guidelines for referral to PHC are based on a visual acuity threshold of less than 6/18 in the better eye.

**Correct Answer: C. 6/18**