Answer:
The piece of evidence that best supports the grandmother's characterization as cunning and manipulative is:
C. "There was a secret panel in this house,' she said craftily, not telling the truth but wishing that she were."
Explanation:
In "A Good Man is Hard to Find", by Flannery O'Connor, the grandmother is quite a cunning woman. When she wants something, she manipulates those around her to get it. She never simply says she wants it. She comes up with stories and reasons, trying to convince others that her way is the best way. Sometimes, she will simply ignore what others want and do her own will - such as bringing the cat along with the family without anyone knowing.
The lines in letter C show her skills in manipulating. Let's pay attention to the adverb "craftily", which means artfully, slyly, cunningly. Let's also note that she is lying. The grandmother is telling the kids a false story to get them to convince Bailey to go see a certain place.