A) pith
That's a tricky question! I immiediately thought the answer was D, but it asks about the CENTER of the plant, which is the pith.
Explanation:
"After harvesting the Cyperus papyrus plant, the outer fibers are peeled away and the core of the stalk and sliced into very thin strips that are as broad as possible. The best of these strips, from the perspective of quality, comes from the center. Progressively, the quality of the papyrus strips decline as the strips are taken further from the center of the yellowish-white pith."
SOURCE: https://www.egyptian-papyrus.co.uk/how_papyrus_is_made.htm
