**Core Concept**
The Snellen's test is based on the principle of visual acuity, which is the sharpness of vision measured by the ability to distinguish between two points of light at a distance. The test type is designed to assess the minimum angle of resolution, which is the smallest angle between two points that can be perceived as separate.

**Why the Correct Answer is Right**
The correct answer is based on the concept of the nodal point of the eye, which is the point where the image formed by the lens is sharp and inverted. The minimum angle of resolution is determined by the size of the cone cells in the retina, specifically the foveal cone cells. These cells are responsible for central vision and are densely packed in the macula. The minimum angle of resolution is approximately 1 minute of arc, which is equivalent to an angle of 1/60 of a degree. This is the smallest angle that can be perceived by the human eye.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too low. The minimum angle of resolution is not as small as 1/120 of a degree.

**Option B:** This option is incorrect because it is too high. The minimum angle of resolution is not as large as 1/30 of a degree.

**Option C:** This option is incorrect because it is a different unit of measurement. While it is close to the correct answer, it is not the correct unit of measurement.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the Snellen's test is a subjective measure of visual acuity and is influenced by the patient's ability to perceive and report the correct answer. Therefore, it's crucial to standardize the testing conditions and use a reliable and validated Snellen's chart.

**Correct Answer:** **C. 1 minute of arc**.