(a) n=4,l=1 (p-subshell), so 4p(b) n=4,l=0 (s-subshell), so 4s(c) n=3,l=2 (d-subshell), so 3d(d) n=3,l=1 (p-subshell), so 3pAccroding to the Bohr (n+l) rule,Enery order of the subshell : 3p<4s<3d<4pNote: When two orbital have the same value of (n+l) then the orbital which have lower value of n will be filled first.