a. 3d
The electrons in an atom fill the shells in order from least energy to higher energy. The orbitals from lowest to highest are
1s - 1 orbital, 2 electrons
2s - 1 orbital, 2 electrons
2p - 3 orbitals, 6 electrons
3s - 1 orbital, 2 electrons
3p - 3 orbitals, 6 electrons.
4s - 1 orbital, 2 electrons. Note the energy for the 4s orbital is just barely
lower than the 3d orbitals. So it fills up before the 3d orbitals.
3d - 5 orbitals, 10 electrons.
Since the 20th electron fills the 4s shell, the 21st electron goes into the next higher energy orbital which is one of the 3d orbitals.
