The correct option is C 8, 8, 8
Electronic configuration of X = 2, 8, 8 = 1s22s22p63s23p6
Electronic configuration of X = 2, 8, 8, 1 = 1s22s22p63s23p64s1
Electronic configuration of X = 2, 8, 8, 2 = 1s22s22p63s23p64s2
M shell is the third orbit of any element i.e. n = 3 and it contains maximum of 18 electrons
In element X, Y and Z, M-shell contains 8 electrons each.