lanthanide series?

Question

the elements with in the lanthanide series show some irregularity in the electron filling order because

1. the near degneracy in the energy levels of the 6s and 5d orbitals
2. the near degneracy in the energy levels of the 6s, 5d and 4f orbitals
3. the near degneracy in the energy levels of the 5f, 6d and 7s orbitals
4. the near degneracy in the energy levels of the 5d and 4f orbitals

Answer 1

2. The near degeneracy of the 6s, 5d, and 4f orbitals.