Correct Option
Option (b):
Assertion (A) states that the weight of a body decreases with an increase in altitude on Earth. This statement is true. The weight (W) of a body is given by the product of its mass (m) and the acceleration due to gravity (g), i.e., W = mg. The acceleration due to gravity (g) is inversely proportional to the square of the distance from the center of the Earth. As altitude increases, the distance of the body from the Earth's center increases, which leads to a decrease in the value of 'g' and consequently a decrease in the body's weight.
Reason (R) states that the Earth is not a perfect sphere. This statement is also true. The Earth is an oblate spheroid, meaning it is flattened at the poles and bulges at the equator. This shape results in variations in the acceleration due to gravity at different latitudes.
However, Reason (R) is not the correct explanation for Assertion (A). The primary reason for the decrease in weight with increasing altitude is the inverse square law of gravitation, which dictates that gravitational force diminishes with increasing distance from the Earth's center. While the Earth's oblate shape causes variations in 'g' based on latitude, it does not explain the fundamental principle of 'g' decreasing with altitude. The decrease in weight with altitude is a direct consequence of moving farther away from the Earth's center of mass.
Incorrect Options
Option (a) is incorrect because although both Assertion (A) and Reason (R) are true, Reason (R) does not provide the correct explanation for Assertion (A). The decrease in weight with altitude is due to the inverse square law of gravitation, not primarily the Earth's oblate shape.
Option (c) is incorrect because Reason (R) is a factually correct statement about the Earth's actual shape.
Option (d) is incorrect because Assertion (A) is a factually correct statement regarding the variation of weight with altitude.