A, Xiao Fang's descent is only influenced by gravity, and there is downward gravity acceleration, so it is in a state of complete weightlessness, so A is wrong.

B, Xiao Fang is in the process of rising after taking off, and is only affected by gravity, with downward gravity acceleration, in a state of complete weightlessness, so B is wrong.

C. When the small square takes off, the ground should give people an upward supporting force, which is greater than human gravity, so people can have upward acceleration and upward movement, so C is correct.

D. According to the theorem of kinetic energy, the work done by the ground on people is finally transformed into the potential energy of human gravity. In this process, people's center of gravity rises by about 1m, so the increased gravitational potential energy is about mgh = 50×10×1j = 500 j, so d is wrong.

So choose C.