Force of gravity is least at

The correct option is The equator.

To understand why the force of gravity is least at the equator, we need to look at two primary factors: the shape of the Earth and the Earth's rotation.

1. Shape of the Earth:
The Earth is not a perfect sphere; it is an oblate spheroid. Due to its rotation, it bulges at the equator and is flattened at the poles. Consequently, the equatorial radius
$R_e$
is greater than the polar radius
$R_p$
by about 21 kilometers.

The force of gravity ($F$) acting on a mass ($m$) at the Earth's surface is given by Newton's law of universal gravitation:
$F=\frac{G\cdot M\cdot m}{R^2}$
where $G$ is the gravitational constant, $M$ is the mass of the Earth, and $R$ is the distance from the center of the Earth to the object on the surface.

Since the force of gravity is inversely proportional to the square of the radius ($F\propto \frac{1}{R^2}$), a larger radius at the equator results in a weaker gravitational pull compared to the poles.

2. Centrifugal Force due to Rotation:
As the Earth rotates on its axis, it generates an outward centrifugal force. This centrifugal force is maximum at the equator and zero at the poles. Since the centrifugal force acts in the opposite direction to the gravitational force drawing objects toward the center, it further reduces the net effective force of gravity at the equator.

Combining both factors, the net force of gravity is greatest at the poles and least at the equator.