Hi Emma! I'm sorry, I wrote that answer quickly and didn't get a chance to finish it before I received an error message and lost forum access. I didn't mean to be confusing, but I had more to say!
Yes, answer choice D is voting by telephone and not present, but they still exist and are not imagined. Whereas, C doesn't matter is it is present or not, because it is an imagined target group. There is no evidence of the actual, concrete existence of the audience in answer choice c.
Think about what this question is asking: Which group listed below is an example of the way the author uses the term fictional audience? You know a fictional audience is imagined. D, though it isn't present, is voting by telephone, so there is evidence of their existence, merely by the fact that they are voting. C we don't have evidence of their existence, and by saying we have a target of that age group, we are "imagining" the possible audience we want to target.