b) communicating something. If it's this, then I strongly suspect that McGonagall is cooperating with future Harry in some rescue plan. She might be communicating a simple message like "Don't worry" or "We'll get you out" which would imply that she has some extra knowledge about how things are going to play out.

Extra supporting evidence for this: McGonagall isn't the sort of person who would give false reassurance. If she didn't see any way to get Hermione out of this, she would say "keep strong." If she's saying "don't worry" then Hermione has some reason not to worry.