A nimbus is a halo around a saint or supernatural being. A halo is a nimbus.
I would pick the first one. We done don't very much about some of the others.
If she saw a nimbus around him, she is giving him positive qualities that he did not have before. This looks like your answer. A is correct.
In this passage, we don't know anything about her secrets or her letters. B is not right.
We don't know (from this passage) that Granny and Doc were lovers. I wouldn't pick C.
Is she angry? We don't know that especially with that nimbus hanging around.