The effect of tone on the reader is best described by this statement: the tone helps the reader interpret an author's attitude on a subject (C).
The tone is the writing style through which the reader can perceive the author's mood, emotion, or opinion about something.
It gives an indication about how the author feels about a subject, more than about the plot itself.
For example, if the author's tone is very sarcastic and/or dubious, you can probably assume that he is not taking the subject very seriously.