In a many-electron atom, the energy order of subshells within the third shell is $3s<3p<3d$ whereas in a hydrogen atom all three are equal in energy. The best reason is:

1. Aelectron-electron repulsion is absent in the $3s$ subshell
2. B$3s$ penetrates the inner shells more than $3p$ and $3d$, so it is less screened and feels a higher $Z_{\mathrm{eff}}$
3. C$3d$ has the highest value of the principal quantum number
4. D$3s$ has the largest number of radial nodes and therefore the highest energy