Aprincipal quantum number refers to:
a the numbered energy levels starting from the nucleus outward
b the total energy of an electron
c the maximum number of electrons at a given level
d the s, p, d, f levels

a) the numbered energy levels starting from the nucleus outward

Explanation:

Principle Quantum Number: It describes the size of the orbital and the main energy level. It is represented by n. Where, n = 1,2,3,4....stands for K, L, M, N shell and so on..

Azimuthal Quantum Number : It describes the shape of the orbital. It is represented as 'l'. The value of l ranges from 0 to (n-1). For l = 0,1,2,3... the orbitals are s, p, d, f...

Magnetic Quantum Number : It describes the orientation of the orbitals. It is represented as . The value of this quantum number ranges from . When l = 2, the value of will be -2, -1, 0, +1, +2.

Spin Quantum number : It describes the direction of electron spin. This is represented as The value of this is for upward spin and for downward spin.