Why are the shells in which electrons revolve around the nucleus of an atom called energy levels ?

electron revolving in a shell is associated with a definite amount of energy. The energy changes when it jumps to another shell that is so say when it goes from one level of energy level to another. Thus shell also gives the energy of a electron besides its location and are therefore called energy levels.

beta decay is the answer

i think   the answer is c hope i'm right