Why C? And why not the others?

Correct answer: C. The Rosetta Stone

This granodiorite stele, carved in 196 BCE, records a priestly decree honouring Ptolemy V in hieroglyphic script, in the everyday Demotic script, and in Ancient Greek. Because scholars could already read Greek fluently, they could work backward line by line, and in 1822 Jean-François Champollion used that comparison to finally crack the phonetic principles behind hieroglyphic writing, ending centuries of failed guesswork. The Narmer Palette is a genuinely important, far older Egyptian carved slate commemorating the unification of Egypt, but it carries no parallel translation and taught scholars nothing about reading the script. The Behistun Inscription played an almost identical trilingual role for cuneiform, unlocking Old Persian, Elamite, and Babylonian rather than Egyptian writing. The Dendera Zodiac is a carved ceiling relief depicting the night sky, prized for its astronomy rather than any role in translation.

Source: Britannica: Rosetta Stone; The British Museum (collection record)