Final answer:
Option (C). Gold is the metal that is so malleable it can be beaten into a 300 ft² sheet from a single ounce and so inert that ancient jewelry still appears new today.
Step-by-step explanation:
The metal in question is gold (Au), which is known for its exceptional malleability and inertness. Gold can indeed be beaten into extremely thin sheets, and due to its chemical stability, it does not react with most substances. This inertness allows gold artifacts to remain unchanged for millennia, hence ancient jewelry still appears lustrous and new today. Therefore, the answer is C) Gold.