Wow. You said so many things that are wrong I do not even know where to start.
First, you ignored half of Jorge's argument. Jorge did not reach a conclusion ("you won't be able to write well about rock music in the 1960's") based upon just one premise ("since you were just an infant then."). There is a second part to the premise (it "was created by and for people who were then in their teens and early twenties"). So, Jorge, besides giving us the what, also gives us the why.
The next thing wrong with your statement is, "[n]othing in Jorge's argument goes assumed, or is unstated." Of course there is. In fact, there is no argument in the world in which there are not unstated facts or assumptions. This one certainly has many. The main assumption is that there is something that ties the reason (created by and for people who were then in their teens and early twenties) to the conclusion ("you won't be able to write well about" it). (For example - that something can only be written about by the people for whom it was intended).
Next we get to Ruth. While you allege that Ruth only addressed the first part of Jorge's argument, she did not. She did not respond by simply stating, writers who were not then born still write well about the Romans. She added a contradiction to the second part of the argument - those writers were not part of the culture - to counter the second half of Jorge's argument.
Did the fact Ruth put forth really counter Jorge's argument? It depends on what the assumption is in Jorge's argument. Jorge is talking about who the music was written by and for. Ruth's example is about people who are not part of the culture. Doesn't matter. Ruth is using an analogy to counter what she believes is Jorge's assumption. That makes Choice D the completely perfect answer.
Choice C is completely wrong. The first parts states, "she uses an example of classic culture . . . ." I do not agree with that. I would agree if it said, "she uses classic culture as an example . . . ." But, she is not using an example of classic culture. That would be something different. While I probably would not automatically reject a response option on these grounds alone, the way the use of the analogy is stated in Choice D is far superior.
The second half of Choice C is even more wrong. This half of the response option says Ruth's response is to "legitimize contemporary culture as an object worthy of serious consideration." She is not doing either of these things. She is not trying to legitimize the culture. Neither party even speaks to that issue. Nor is she discussing whether it is worthy of serious consideration. She is only discussing whether she can write about it.