In which `5f` subshell is half-filled?

In inner transition elements, which f subshell gets filled up with electrons ?

Out of 4s and 3d, which subshell is filled first and why?

Which one of the following d-block elements has half-filled penultimate d-subshell as well as half-filled valence s-subshell

Which one of the following d-block elements has half-filled penultimate d-subshell as well as half-filled valence s-subshell?