I got this question wrong and I believe it's because I misunderstood the question.
Does the question asking what principle the author is REFERRING TO when he/she mentions "existing generic parameter"?
If so, then I guess answer choice (A) makes perfect sense.
I read the word "include" in the question and interpreted it as what NEW PRINCIPLE the author WOULD LIKE to include to this exisitng generic parameters.
So I ended up choosing E over D.
Now that I think about it, I think it was a dumb, time consuming move. But I would I appreciate some clarification. Thanks!