Assertion (A) : To orbit around the Sun, the planet Mars takes lesser time than the time taken by the Earth. Reason (R) : The diameter of the planet Mars is less than that of the Earth.

The assertion is false: Mars takes about 687 Earth days to orbit the Sun, which is longer than Earth's ~365-day sidereal period; thus Mars does not take lesser time than Earth to orbit the Sun. The reason statement is true in isolation: Mars has a smaller diameter than Earth. However, the reason does not explain the assertion. Orbital period depends on the semi-major axis of the planet's orbit (Kepler's third law: T^2 ∝ a^3), not on the planet's physical diameter; a planet farther from the Sun has a longer period regardless of its size [1]. Therefore A is false while R is true, but R is not the explanation for A.

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 2: The Solar System > Kepler's Laws of Planetary Motion > p. 21