Structure of Atom Part 1

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