The correct order of energy for orbitals in a multi-electron atom is:

Given below are two statements:
Statement-$I$: The orbitals having same energy are called as degenerate orbitals.
Statement-$II$: In hydrogen atom,$3p$ and $3d$ orbitals are not degenerate orbitals.
In the light of the above statements,choose the most appropriate answer from the options given.

The hydrogen-like species $Li^{2+}$ is in a spherically symmetric state $S_1$ with one radial node. Upon absorbing light,the ion undergoes transition to a state $S_2$. The state $S_2$ has one radial node and its energy is equal to the ground state energy of the hydrogen atom.
$1.$ The state $S_1$ is
$(A)$ $1s$ $(B)$ $2s$ $(C)$ $2p$ $(D)$ $3s$
$2.$ Energy of the state $S_1$ in units of the hydrogen atom ground state energy is
$(A)$ $0.75$ $(B)$ $1.50$ $(C)$ $2.25$ $(D)$ $4.50$
$3.$ The orbital angular momentum quantum number of the state $S_2$ is
$(A)$ $0$ $(B)$ $1$ $(C)$ $2$ $(D)$ $3$
Give the answer for questions $1, 2$ and $3.$

If $n + l = 6$,then the total possible number of subshells would be

The number of unpaired electrons in the $p$-subshell of oxygen atom is

The two electrons occupying the same orbital are distinguished by: