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.

Analysis of Assertion (A): Assertion (A) states that Mars takes lesser time than Earth to orbit the Sun. This is incorrect. Earth's orbital period is about 365 days, while Mars's orbital period is about 687 Earth days. Planets further from the Sun have longer orbital periods, as described by Kepler's Third Law. Since Mars is further from the Sun than Earth, it takes longer to complete one orbit. Therefore, Assertion (A) is false. Analysis of Reason (R): Reason (R) states that the diameter of Planet Mars is less than that of the Earth. This is correct. Earth's diameter is approximately 12,742 km, while Mars's diameter is approximately 6,779 km. Therefore, Reason (R) is true. Conclusion: Since Assertion (A) is false and Reason (R) is true, the correct option is D. The final answer is D because Assertion (A) is false (Mars takes longer to orbit the Sun than Earth), but Reason (R) is true (Mars's diameter is indeed less than Earth's diameter).