Sally tells engaging stories, but when she uses them in her speeches they are rarely effective because they do not "ring true" with what the audience already knows about her topic. her use of narrative violates what basic principle

The correct answer to this is the:

"Fidelity"

I believe Sally is violating fidelity simply just to make her speeches more good sounding. Fidelity is described as being faithful or being true to other people. By adding, subtracting or modifying some parts in her speeches just to be more entertaining, she is not telling the real story at all.