The scholars knew that the inscription was the same in all three languages on the Rosetta Stone because the last sentence of the Greek text explicitly stated, "This decree shall be inscribed on a stela [slab] of hard stone in sacred [hieroglyphic] and native [demotic] and Greek characters." This clear declaration indicated that the content of the inscription was identical across the Greek, hieroglyphic, and demotic texts, even though the scholars had not yet fully deciphered the hieroglyphs or demotic writing. Thus, the correct answer is:
The last sentence explained that the inscription was the same.