FALSE is correct in the context of a hydrogen atom.

In the hydrogen atom, the energy levels of electrons are determined primarily by the principal quantum number $n$. For a given value of $n$, all sublevels (s, p, d, and f) have the same energy. This property is a consequence of the hydrogen atom's simplicity and the fact that its energy levels depend only on the principal quantum number due to its one-electron system. Therefore, the energy of an electron in a 3d-orbital ($n=3$) is the same as the energy of an electron in a 3s or 3p orbital in a hydrogen atom.

However, in multi-electron atoms, the energy levels are split due to electron-electron repulsions and the overall greater complexity of the quantum states. This is where the energy of an electron in the 4s-orbital can be lower than that in the 3d-orbital, making the 4s level fill before the 3d in the process of electron configuration in multi-electron atoms such as potassium or calcium. But, in the specific case of the hydrogen atom, which contains only one electron, the statement that the energy of the electron in the 3d-orbital is less than that in the 4s-orbital is incorrect, thus FALSE.