What is the maximum number of electrons that call occupy n = 3 energy level?

 The maximum number of electrons that call occupy n = 3 energy level is 18.

The most electrons that will fit inside of a "n" shell is $2{\mathrm{n}}^2$.

No of electron $=2\times 3^2$

No of electron$=2\times 9$

No of electron $=18$

The shell, not the orbital, is defined by the fundamental quantum number n. Each of the many orbitals that make up a shell may hold up to two electrons, one of which spins counterclockwise and the other clockwise.

Hence, the option D is correct.