The correct answer is: C. 6,400 km.
The mean radius of the Earth is the average distance from the center of the Earth to its surface. It is approximately 6,371 kilometers (3,959 miles). The Earth is not a perfect sphere, but is slightly flattened at the poles and bulges at the equator. This is due to the centrifugal force created by the Earth’s rotation.
Option A is incorrect because it is the diameter of the Earth, not the radius. The diameter of the Earth is approximately 12,756 kilometers (7,926 miles).
Option B is incorrect because it is the radius of the Moon, not the Earth. The radius of the Moon is approximately 1,737 kilometers (1,079 miles).
Option D is incorrect because it is the radius of the Sun, not the Earth. The radius of the Sun is approximately 696,000 kilometers (432,000 miles).