Maximum number of electrons that can be present in $M$ and $N$ shells respectively are:

The correct option is A $18,32$
Maximum number of electrons that can be present in M and N shells are 8,18 respectively.
M shell corresponds to $n=3$. So, $2n^2$ and total 18 electrons are possible in M shell.
N shell corresponds to $n=4$. It has 4s, 4p and 4d subshells. s subshell has 2 electrons, p subshell has 6 electrons and d subshell has 10 electrons. . So, $2n^2$ total 32 electrons are possible in N shell.