The correct definition of density is the degree of compactness of a substance.
Sun emits lights
moon reflects light
lamp emits light
cloud reflects
mirror reflects light
firefly emits light
lightning emits light
tv set emits light
Because the 4s orbital is at a lower energy level.
The 4s orbital has a lower energy because it is more <em>penetrating</em> than a 3d orbital. A 4s electron can get closer the nucleus than a 3d electron, so it has a lower potential energy.
The other options are incorrect because they contradict the correct one.