When considering the acceleration due to gravity (free fall acceleration) at sea level, it is important to understand the shape and structure of the Earth.

Option-1 states that the Earth is a sphere with the same density everywhere. While it is true that the density of the Earth is relatively consistent, this does not directly explain why the free fall acceleration increases as one moves from the equator towards either pole.

Option-2 suggests that the Earth is a sphere with different density at the polar regions compared to the equatorial regions. However, density alone does not determine the variations in free fall acceleration across the Earth`s surface.

Option-3 correctly explains that the Earth is approximately an ellipsoid, meaning it is slightly flattened at the poles and bulges at the equator. This shape is often referred to as an oblate spheroid. The equatorial radius of the Earth is greater than the polar radius by about 21 km. As a result, the distance from the center of the Earth to sea level is greater at the equator compared to the poles. This increase in distance causes a decrease in gravitational force and a higher free fall acceleration at the poles compared to the equator.

Option-4 is incorrect because the Earth`s equatorial radius is greater