What is the orbital angular momentum of an electron in a $2s$-orbital?

Which of the following sets of orbitals may degenerate?

Which of the following sets of quantum numbers is correct for an electron in a $4f$ orbital?

If the electronic configuration of a nitrogen atom were $1s^7$,its energy would be lower than that of the ground state configuration $1s^2\,2s^2\,2p^3$ due to being closer to the nucleus. However,the $1s^7$ configuration is not observed because it violates which of the following rules?

For $H^{-}$ atom,the energy required for the removal of an electron from various sub-shells is given as under:
The order of the energies would be:

Assertion $(A)$: Atoms with completely filled and half-filled subshells are stable.
Reason $(R)$: Completely filled and half-filled subshells have symmetrical distribution of electrons and have maximum exchange energy.
The correct answer is: