The number of orbitals present in the shell with $n = 4$ 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.

Explain how the energy of an orbital and the extent of shielding in the same shell depend on the shape of the orbitals.

In the ground state, an element has $13$ electrons in the $M$ shell. The element is

Which one of the following sets of quantum numbers represents an impossible arrangement?

$A$ subshell with $n=3$ and $l=2$ can accommodate a maximum of: