Answer:
The central idea of this excerpt is that an almost complete copy was discovered.
Explanation:
The Rosetta Stone was translated into English was not the central idea. Although mentioned, it wasn't central idea. Experts had a new theory but that wasn't still the central idea, so does the new Greek version being discovered.
The passage here mentions the whole idea.
"This makes present-day experts hope that an even more complete copy of the hieroglyphic inscription may yet be found, somewhere in Egypt."