I selected other. I feel that Bob's post's visibility should be defined by Bob's settings for that post.
Followers of Bob that don't also follow Alice could see his reply and know that it was a reply to something else, but they'd not see what it was in reply to.
Conceptual parallel: one can comment publicly on copyright protected material that others may not be able to see. one can also comment publicly on classified info (there may be penalties for doing so, but it can be done).
