(a) 2, 8, 2
(b) 2, 8, 1
(c) 2, 8, 7
(d) 2, 1
Metals usually have 1, 2 or 3 valence electrons. In the given case, a, b and d are metals and are electropositive in nature. These can readily lose electrons to attain nearest noble gas electronic configuration.
(a) is magnesium which has atomic number 12.
(b) is sodium which has atomic number 11.
(c) is chlorine which has atomic number 17.
(d) is lithium which has atomic number 3.