Option 4 is the correct answer, which states that the assertion is false but the reason is true. This means that Mars does not take less time to orbit the Sun compared to Earth, instead it takes longer—about 687 Earth days. Hence, the assertion is false.

For the reason provided, it is true, as Mars does indeed have a smaller diameter than Earth. Mars`s diameter is approximately half of that of Earth`s. However, this fact does not influence the length of a planet`s year, which is determined instead by the distance from the sun and the speed at which a planet orbits around the sun.

Therefore, each statement needs to be evaluated individually, and the connection between the two (if any) should be correctly interpreted. Thus, despite `R` being true, it does not correctly explain `A`.