The correct option is D Effect of gravity on the motion of the molecules is negligible.
According to postulates of KTG :
1. Gas contains large number of molecules which are identical and the volume occupied by the molecules is negligible compared to the volume of container.
2. The molecules are in continuous random motion. This random motion is also known as Brownian motion.
3. Collisions made by the molecules with the walls of container are perfectly elastic.
4. Effect of gravity on the motion of the molecules is negligible.
Hence, options (a),(b), & (d) are correct.