6371 km
Explanation
The Earth is not a perfect sphere but an oblate spheroid (Geoid), meaning it is slightly flattened at the poles and bulges at the equator. Consequently, the distance from the center of the Earth to the surface varies depending on the location.
Detailed Analysis:
- Equatorial Radius: Due to the centrifugal force generated by the Earth's rotation, the radius at the equator is approximately 6,378 km.
- Polar Radius: The distance from the center to the poles is shorter, approximately 6,357 km.
- Mean Radius: For general scientific and navigational calculations, the Earth's mean radius is globally accepted as approximately 6,371 km.
Incorrect Options Analysis:
- 3500 km 3500 km: This figure roughly corresponds to the radius of the Earth's Core (approx. 3,485 km) or the radius of Mars (approx. 3,390 km).
- 2900 km 2900 km: This represents the approximate depth of the boundary between the Earth's Mantle and the Outer Core (Gutenberg Discontinuity).
- 6000 km 6000 km: This is an inaccurate approximation that significantly underestimates the Earth's actual size.
Key Takeaway:
The Earth is an oblate spheroid with a mean radius of 6,371 km, varying slightly between the equator (6,378 km) and the poles (6,357 km).