**Core Concept**
The National Programme for Control of Blindness (NPCB) has implemented a screening strategy to detect diabetic retinopathy, a microvascular complication of diabetes mellitus, which can lead to blindness if left untreated. This strategy focuses on early detection and treatment to prevent vision loss.

**Why the Correct Answer is Right**
The correct answer is based on the recommendation of the NPCB to screen diabetic patients for retinopathy using **digital retinal imaging** (also known as **retinal photography**). This non-invasive technique captures high-resolution images of the retina, allowing for the detection of early signs of diabetic retinopathy, such as microaneurysms, hemorrhages, and hard exudates. The images are then evaluated by a trained ophthalmologist or diabetologist to identify any abnormalities.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because **visual acuity testing** alone is not sufficient to detect diabetic retinopathy, as it only measures central vision and does not assess the retina.
* **Option B:** This option is incorrect because **fluorescein angiography** is an invasive procedure that involves injecting a dye into the bloodstream, which is not a practical or cost-effective screening method for diabetic retinopathy.
* **Option C:** This option is incorrect because **optical coherence tomography (OCT)** is a diagnostic tool that provides detailed cross-sectional images of the retina, but it is not a screening method for diabetic retinopathy.

**Clinical Pearl / High-Yield Fact**
The NPCB recommends that all diabetic patients undergo retinal screening at least once a year, starting 5 years after the diagnosis of diabetes.

**Correct Answer:** C. Optical Coherence Tomography (OCT)