Options A and B both look good with a slight preference for A. There is so much unused real estate there. It'd be nice to also include meta (and while on meta, SO). I find it really awkward trying to get to it from the menu bar.
Update:
I just experienced option C. It seems terrible. It's a bit hard to understand why you would give over the absolute most prime real estate on site X to promoting sites Y, Z and W. Even the collapsing isn't enough - now there's a big chunk of completely non-functional wasted space, again, right in the middle of prime real estate:
If this becomes a thing, I'll definitely use UBlock Origin to remove it. (Just like I removed the "hot network questions" which are impossibly distracting for me.)
(Unrelated: dark mode is awesome. Thank you!)