Explanation
The correct answer is (a) Geoid.
The shape of the Earth is best described as a geoid. The geoid is a three-dimensional representation of the Earth's gravitational field, taking into account the irregularities in the distribution of mass across the planet. It is an approximation of the Earth's true shape, which is not perfectly spherical or ellipsoidal.
While the Earth is often simplified as a sphere for many purposes, in reality, it is slightly flattened at the poles and bulging at the equator due to its rotation. This deviation from a perfect sphere is due to the centrifugal force generated by the Earth's rotation.
The geoid is the most accurate representation of the Earth's shape, considering the complex interactions between gravity, rotation, and mass distribution. It serves as a reference surface for determining elevations, calculating accurate satellite orbits, and understanding the Earth's gravitational field.
Therefore, the correct answer is (a) Geoid.
