Assertion (A) : The weight of a body decreases with the increase of latitude on earth Reason (R) : The earth is not perfect sphere

Assertion is false: measured weight (apparent g) increases toward higher latitudes (is greater at the poles and smaller at the equator), so weight does not decrease with increasing latitude [1]. Reason (R) is true: the Earth is not a perfect sphere but an oblate spheroid with an equatorial bulge produced by rotation [2]. The observed latitude variation of weight arises from two effects: larger equatorial radius (greater distance from Earth’s center reduces gravitational attraction) and centrifugal acceleration from Earth’s rotation (maximum at the equator, reducing apparent weight). These effects make weight smaller at the equator and larger at the poles, so while R is correct, it does not make A true—indeed it helps explain the opposite trend [2].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 18: Latitudes and Longitudes > The Shape of The Earth and Latitudinal Heat Zones > p. 241
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 7: Tectonics > Forces Behind The Drifting Of Continents (According to Wegener) > p. 95