How are atomic orbitals (electrons) identified or distinguished?

The correct set of four quantum numbers for the valence electron of rubidium $(Z = 37)$ is:

Which of the following statements are correct?
$(a)$ When an electron in an atom moves from $n=4$ to $n=2$,it gains energy.
$(b)$ An electron in a $3p$ orbital would have less energy than an electron in a $2p$ orbital.
$(c)$ $A$ $3s$ subshell has a maximum of $2$ electrons.
$(d)$ $A$ $3^{rd}$ orbit may contain a maximum of $10$ electrons.

The orbital angular momentum of a $3p$ electron is

What is the maximum number of electrons that can be accommodated in a single $p$ orbital?

Explain the stability of completely filled and half-filled subshells by giving reasons.