Explanation:

- Historically, the Indian rupee was primarily a silver coin.

- ?? Option 3: Silver

- Silver coins were widely used for trade and commerce in India.

- The Mughal emperor Sher Shah Suri introduced the silver rupee in the 16th century.

- This standard continued with variations until the 20th century.

- Option 1: Copper

- Copper coins were used in smaller denominations but were not the main unit.

- Option 2: Gold

- Gold coins did exist but were used for ceremonial or high-value transactions.

- Option 4: Bronze

- Bronze was used more for lower denominations, not for main currency.