After completely filling the $2s$ subshell, the electrons must occupy the next lowest-energy subshell, which is the $2p$ subshell and not the $3s$ one.
Work Step by Step
$1s^22s^23s^1$ As we have learned, the electrons would try to occupy the subshell with the lowest possible energy for maximum stability. - After the orbital in $1s$ subshell is completely filled with 2 electrons, the electrons move to the next lowest-energy subshell, which is the $2s$ subshell. Until now, things are correct. - After the orbital in $2s$ subshell is completely filled with 2 electrons, however, the next lowest-energy subshell that the electrons will occupy is the $2p$ subshell, not the $3s$ subshell. - Therefore, the correct configuration must be $1s^22s^22p^1$.