The shape of the earth is

Answer: (c) Geoid

Earth is a geoid — an oblate spheroid, flattened at the poles, bulging at the equator.

The idea

Earth is roughly spherical but rotation makes it bulge at the equator and flatten at the poles. An oblate spheroid is a smooth mathematical approximation of this shape. A geoid is the uneven surface of equal gravitational potential that approximates mean sea level, extended beneath land.

Earth’s gravity gives a broadly round shape → Rotation produces an equatorial bulge → An oblate spheroid models the broad shape → Uneven mass distribution changes gravity locally → The geoid follows the resulting mean-sea-level reference surface

Why the others are wrong

(d) Not a perfect sphere.

Also know

Topography, the reference ellipsoid and the geoid describe different surfaces. Mountains and valleys belong to topography; the ellipsoid is used for coordinates; the geoid is used as a reference for heights. The geoid is not simply the outline of mountains and ocean trenches.