Which sub shell in the fourth transition series is in the process of completion?

Which sub shell in the fourth transition series is in the process of completion?

Explanation
In the fourth transition series of the periodic table, the 6d subshell is in the process of completion.
This series includes elements with atomic numbers starting from
Actinium (Ac, 89) and extending to element 112, with the involvement of the 6d orbitals.