or a given principal level n = 4, the energy of its subshells is in the order

The correct option is s < p < d < f.

To understand why this order is correct, we can look at the factors that determine the energy of subshells within a given principal quantum shell (where the principal quantum number is $n$).
For a multi-electron atom, the energy of an orbital (or subshell) depends on both the principal quantum number ($n$) and the azimuthal quantum number ($l$). This is described by the Bohr-Bury $n+l$ rule:
1. Subshells with a lower value of $n+l$ have lower energy.
2. If two subshells have the same value of $n+l$, the one with the lower value of $n$ has lower energy.

For a given principal quantum level $n=4$, we have four subshells corresponding to different values of the azimuthal quantum number $l$:
• For the s subshell: $l=0$, so $n+l=4+0=4$
• For the p subshell: $l=1$, so $n+l=4+1=5$
• For the d subshell: $l=2$, so $n+l=4+2=6$
• For the f subshell: $l=3$, so $n+l=4+3=7$

Comparing the $n+l$ values, we get:
$4<5<6<7$
Therefore, the energy of the subshells increases as the azimuthal quantum number $l$ increases.

This results in the following energy order for the subshells of the $n=4$ shell:
s < p < d < f