Assertion : An astronaut experience weightlessness in a space...

Both the statements of assertion and the reason are correct. They are independent statements. The statement given in the reason is not the reason for the weightlessness of the astronaut. The centripetal and centrifugal forces cancel out. In the case of free fall, the pseudo acceleration, $g$ acts upwards. The body does not feel it. But all the same it will fall down to the earth. His mind which feels, is riding the lift of his body. The body is falling down with acceleration $g$. In the first case, the force of acceleration due to gravity is cancelled by the centrifugal force acting outwards. Both the statements are correct but the reason given is not the reason for the assertion.