The official answer is a) because the attributes of the line can be derived from its membership of the boundary relation. This seems to be good enough for the standard renderer to do the right thing. But I believe adding the extra tags is superfluous, but not actually wrong. It’s similar to access=yes or oneway=no. I hate it when people share the boundary way with tags for waterway, highway etc - it’s a maintenance nightmare. The boundary ways should exist in their own right, and therefore some top-level tag would seem appropriate to me.
I personally prefer c) because my editor of choice (Potlatch2) styles the ways only according to the tags on the way, which leads to a spaghetti of narrow black lines. If the boundary ways at least have boundary=administrative on them then they are distinguishable during editing.
This may be a problem for other people as well - I frequently see cases where footpaths (for example) and boundaries have obviously been confused during editing.
