The correct option is D Assertion is incorrect but Reason is correct
All gas molecules do not move with same speed. They move with ever changing speed and ever changing direction due to frequent collisions with each other and with walls of the container.
Thus, the assertion is incorrect.
Speed of molecules in a gas follows Maxwell's distribution law. According to this law, at a given temperature,
(a) maximum number of molecules of the gas has maximum speed, called most probable speed.
(b) number of molecules with too high or too low speed is very small.
(c) no molecule has zero speed.
Thus, the reason is correct.