(d) 2, 8, 1.
Option (a): Atomic number of sodium is 11. Hence its atomic configuration also must have 11 electrons but 2,8 has only 10 electrons hence this option is incorrect.
Option (b): K shell can have maximum of 2 electrons but in this option K shell has 8 electrons .Hence it is incorrect.
Option (c): L shell can have maximum of 8 electrons and it needs to be filled first before filling M shell but in this option L shell has only 1 electron and M shell is filled before filling L shell. Hence this option is incorrect.
Option (d): since the atomic number of sodium is 11 and K, L, M shells can have maximum 2, 8, 18 electrons respectively. Therefore the correct electronic configuration of sodium is 2, 8, 1 where K shell has 2 electron, L shell has 8 electrons, M shell has 18 electrons. Therefore this option is correct.
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