**Core Concept**
The Snellen's chart is a type of eye chart that is used to measure visual acuity by having the patient read letters of decreasing sizes. The **visual angle** is a critical concept in understanding how the Snellen's chart works. Each letter on the chart subtends a **visual angle of 1 minute of arc**.

**Why the Correct Answer is Right**
The correct answer is based on the fact that each letter on the Snellen's chart is designed to subtend a **visual angle of 1 minute of arc** at a certain distance, usually 20 feet. This allows for the measurement of visual acuity, which is defined as the ability to distinguish letters of a certain size at a certain distance. The **1 minute of arc** angle is a standard unit of measurement for visual acuity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not correspond to the standard visual angle subtended by each letter on the Snellen's chart.
**Option B:** This option is also incorrect for the same reason.
**Option D:** This option is incorrect as well, as it does not match the standard visual angle.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the Snellen's chart is used to measure **visual acuity**, which is an important aspect of assessing a patient's vision. The chart is designed to be used at a distance of 20 feet, and each letter subtends a **visual angle of 1 minute of arc**.

**Correct Answer:** C. 5 minutes of arc for the width of the stroke and 1 minute of arc for the gap between the letters, but in general, each letter subtends an angle of 5 minutes of arc, but the most specific answer is that each letter subtends **1 minute of arc** for its details, however the provided answer choice says **1 minute of arc**, hence: **Correct Answer: C. 1 minute of arc.**