Hi Ellie90
It is True B, C & D would all work but the question is about Best Practice. In terms of Best Practice the thing that immediately stands out for me is the Connector Pane used in answer B. The reason it stands out for me is the consistency it will provide. The 4x2x2x4 (used in B) is the one most commonly used Connector Pane in LabVIEW. Calling the Timer VI as a SubVI with the Connector Pane shown in B means the Timer VI will most likely have the same Connector Pane as the VIs around it which would minimise unnecessary wire bends. I also like that B (and D) has a few terminals that are not allocated, this gives some flexibility for the VI to expand. If I had to choose though, I would go for Connector Pane first, spare terminals second.
Hope this helps
Steve