What is the radius of the earth? (a) 6317 km (b) 6370 km (c) 6371 km (d) 6731 km

Answer: option (D) 6731 km

Explanation: A global average of 6,371 km (3,959 miles) is typically expected with a 0.3% (± 10 km) variation for the following reasons:

The International Union of Geodesy and Geophysics (IUGG) provides three reference values. The average radius (R1) of the three radii was measured at two equatorial points and one pole.
A real radius is the radius of a sphere of equal surface area (R2).
Volume radius is the radius of a sphere that has the same volume as an ellipsoid (R3). All three values are approximately 6,371 kilometres (3,959 miles).
Other methods of defining and measuring the radius of the earth include the radius of curvature.
Some definitions produce values outside the range between the polar and equatorial radii. This is because it contains local or geoid terrain, or because it relies on abstract geometric considerations.
The equator radius, or semi-major axis of the earth, is the distance from its centre to the equator, which is 6,378.1370 km (3,963.1906 mi). The equatorial radius is often used to compare the Earth to other planets.