Identify what “sixth period” means

The 6th period of the periodic table starts from Cs $(Z=55)$ and ends at Rn $(Z=86)$.

Write the order of filling relevant to the 6th period

In the 6th period, electrons enter orbitals in the following sequence: $6s \rightarrow 4f \rightarrow 5d \rightarrow 6p$

This is seen from the elements:

• Cs, Ba: filling of $6s$
• Lanthanoids: filling of $4f$
• Some transition elements: filling of $5d$
• p-block elements till Rn: filling of $6p$

Check each option

• A: $6s, 5d, 5f, 6p$
  Incorrect, because $5f$ filling belongs mainly to the 7th period (actinoids), not the 6th period.

• B: $6s, 4f, 5d, 6p$
  Correct, this is exactly the set of orbitals filled in the 6th period.

• C: $6s, 6p, 6d, 6f$
  Incorrect, $6d$ and $6f$ are not filled in the 6th period.

• D: $6s, 5f, 6d, 6p$
  Incorrect, $5f$ and $6d$ are associated with the 7th period region.

Final answer

Therefore, the orbitals filled in the sixth period are: $6s,\ 4f,\ 5d,\ 6p$

So the correct option is B.