Leonid,
One of the trickiest aspects of the PMP exam is that the questions often propose multiple answers that are possibly correct. The mission of the exam taker is to choose the best among the answers proposed.
The answer is D because the change being so impactful on cost and schedule implies something much more significant than an ambiguous requirement or scope statement. If the software was ready to demonstrate, this means that the development team has already worked on refining requirements and producing software for the previously identified work. Remember, this is an experienced team. If something that big was missed, most likely it's because someone with key input regarding the needed product was missed and, hence, their requirements are simply not being met.
FWIW, after 35 years in IT software development, I can sadly confirm that big issues in delivery are often traceable to this very issue!
I hope this helps!
Tracy