(i) Energy of electron depends on sum of n+1 values. (ii) For the same sum of n+1, the orbital with lower value of n is filled first. Lower be the sum, lower be the energy of electron. (iii) Electrons are filled in increasing order of energy. Thus, among the given options, n+1 for : (a) 4+0=4 (b) 4+1=5 (c) 5+0=5 (d) 3+2=5 Hence (a) i.e. 4+0=4, is the correct answer.