Under the given situation for n=1,l=0,1,2 n=2,l=0,1,2,3 n=3,l=0,1,2,3,4 According to (n+l) rule of order of filling of subshells will be: 1s1p1d2s2p3s2d3f Atomic number ls21p4 Atomic number 1s21p61d1 Atomic number 1s21p6 Atomic number 1s21p61d5 Therefore option (b) is correct. Atomic number of first noble gas will be 18(1s21p61d10).