What Do We Mean When We Say That "The Energy

What do we mean when we say that "The energy of the electron in an atom is quantized?"

A) The electron has a very small amount of energy.
B) The energy of the electron is proportional to the mass of the nucleus.
C) When an electron changes its energy, it emits a quantum of light.
D) The energy of the electron can have certain fixed energies and not others.
E) The electron must be a wave.