Answer choice C is simply saying that the success of a play is linked directly to how much the audience can relate the characters. If the audience can relate to the characters 30% more, then the play is 30% more successful. Vice-versa applies as well, and with all increases/decreases regardless how large.
Be wary with assumption answer choices that state two items are directly proportional. Similar to the aforementioned "most important factor" answer choice (which is basically always wrong, as far as I know), this is also almost always wrong.
I hope this helps, please let me know if you have any other questions.