On absorbing energy, an electron moves to an excited state (unstable state) from its ground state (stable state). Since the electron is in unstable state so it tends to rearrange itself to return to its lowest energy state. On returning to lowest energy state which is stable, it releases energy in the form of light.
Hence, option A that is the electron releases energy in the form of light on falling to lower energy state from excite state.
Democritus was the first person to theorize the existence of atoms.
KE=.5mv^2
M=mass
v=velocity
.5(4)(100)=200
That should be the answer.
