What is the maximum number of electrons that the third orbit or M-shell can have?

18

The correct answer is option 3. 18.

The maximum number of electrons that the third orbit or M-shell can have is 18.

The third electron shell, also known as the M-shell, can hold a maximum of 18 electrons. This is based on the maximum number of electrons that can occupy the 3s, 3p, and 3d orbitals within the M-shell according to the rules of electron configuration and the Pauli exclusion principle.