Why C? And why not the others?

Correct answer: C. The Antikythera mechanism

Corroded and fragmentary when divers first found it, this bronze device turned out to house more than thirty interlocking gears once X-ray and CT scanning let researchers see through the encrustation decades later, revealing a mechanism from around the 2nd century BCE that modelled the movements of the sun, moon, and planets, predicted eclipses, and even tracked the four-year cycle of the ancient Olympic Games. Nothing else of comparable mechanical complexity is known to have existed again for well over a thousand years. The Baghdad Battery is a much later clay jar containing a copper cylinder and iron rod whose supposed use as a battery is unproven speculation. The Nimrud lens is a polished rock-crystal object from Assyria that some have speculated was a magnifying lens, again without consensus. The Berlin Gold Hat is a Bronze Age European ceremonial cone believed to encode a calendar, but it is a decorative headpiece, not a geared mechanical device.

Source: Britannica: Antikythera mechanism