Answer:
Ce sont des feuilles de papier.
Step-by-step explanation:
There word feuilles is the subject (thus plural) not that they are more of paper. It's another silly French exercise that you'll need to re-read the question.
on a side note I don't think anyone (except your teacher) would blink an eye if you said C'est des feuilles de papier, but since the "Ce sont des feuilles de papier." was an answer it's 100% right!