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

Explanation

In the fourth transition series, the subshell that is in the process of completion is the 3d subshell. The 3d subshell can hold a maximum of 10 electrons. As electrons fill up the energy levels, they occupy the lower energy levels first before moving to higher energy levels. So, as the 3d subshell fills up, the transition series is progressing towards completion.