The M-shell is the third energy level or shell in an atom. The maximum number of electrons in each shell can be found using the formula 2n^2, where n is the principal quantum number. For the M-shell (n=3), the formula gives us 2(3)^2 = 18.

Option 1: 6 electrons is incorrect. This number corresponds to the maximum number of electrons in the second energy level or L-shell (n=2).

Option 2: 8 electrons is incorrect. This number corresponds to the maximum number of electrons in the first energy level or K-shell (n=1).

Option 3: 18 electrons is the correct answer. According to the formula 2n^2, the M-shell can hold a maximum of 18 electrons.

Option 4: 32 electrons is incorrect. This number corresponds to the maximum number of electrons in the fourth energy level or N-shell (n=4).

Therefore, the correct answer is option 3: 18.