**Core Concept:** The circumference of an adult eyeball is an important parameter to understand the size of the eyeball which has implications in various ophthalmic procedures and measurements.

**Why the Correct Answer is Right:** The correct answer is derived from the geometric properties of a circle. The circumference (C) of a circle can be calculated using the formula: C = 2πr, where 'r' is the radius of the circle. In this case, we are considering the eyeball as a circular structure.

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because it does not follow the correct formula for calculating the circumference of a circle.
B. This option is incorrect for the same reason as A, failing to account for the radius (r) in the calculation.
C. This option is incorrect because it lacks the π (pi) term in the formula, which is essential for calculating the circumference of a circle.
D. This option is incorrect as it does not include the π term in the calculation, making the answer inaccurate.

**Clinical Pearl / High-Yield Fact:** Pi (π) is a mathematical constant approximately equal to 3.14159, representing the ratio of the circumference of a circle to its diameter. This value is widely used in calculating various geometrical measurements, including those of the human eye, such as the corneal curvature and lens thickness.

**Correct Answer:** Circumference of an adult eyeball is approximately 10.8 cm (3.14159 x 3.5 cm).