The 71st electron of an element X with an
atomic number of 71 enters into the orbital

Lutetium is an element X with atomic number 71 and the electronic configuration.$\left[\mathrm{Xe}\right]4{\mathrm{f}}^{14}6{ \mathrm{s}}^{2}5{ \mathrm{d}}^1$.

The last electron will enter the 5d orbital.

Therefore, Option D is the correct answer.