Which among the following elements possesses one electron in the $4s$ orbital in its observed electronic configuration?

From which principal quantum number $(n)$ do the $s, p, d,$ and $f$ orbitals start? Describe them.

Which orbital among the following has zero radial nodes and $2$ angular nodes?

The order of increasing energies of the orbitals follows:

Given below are two statements. One is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$ : Energy of $2s$ orbital of hydrogen atom is greater than that of $2s$ orbital of lithium.
Reason $R$ : Energies of the orbitals in the same subshell decrease with increase in the atomic number.
In the light of the above statements,choose the correct answer from the options given below.

Orbital angular momentum of an electron in $3s$ and $3p$ subshells are: