The correct answer is option 3, Age of the Buddha. Coins made of metal first appeared during the Age of the Buddha. During this time, around the 6th century BCE, there was an increased need for a standardized form of currency for trade and commerce. The use of metal coins allowed for easier exchange and facilitated economic transactions. In earlier civilizations like the Harappan Civilisation (option 1) and the Later Vedic Age (option 2), barter system or commodities were used as a medium of exchange. The Mauryas (option 4), who ruled during the 3rd century BCE, did greatly contribute to the development of coinage in ancient India, but coins had already been in use for several centuries before their reign.