A? N+l gives an idea about energy Im multielectron atom
why not c?
If n+l is equal then the one with higher n is at a higher energy
btw @Chirag_Hegde yeh magnetic aur electric field joh likha hai uska kya mtlb hai ??????
Not sure how electric field affects,
but as magnetic quantum number is orientation with respect to a magnetic field, so it will affect the energy so 0 magnetic field is necessary to compare only based on n+l