Atomic number of chlorine , CL = 17 so, number of ELECTRONS present in Cl atom = 17 so, number of electrons present in Cl⁻ ion = 17 now, configure based of SHELLS = 2 , 8, 7
we know, K contains 2 electrons. L contains 8 electrons. M contains 18 electrons.
so, in Cl⁻ ion K and L are complete shells and M is INCOMPLETE.