Detailed Solution:

The correct answer is “Copper”.

Copper is the first metal known to have been used by humans.

Its use dates back to the Chalcolithic Age (Copper Age), which marks the transitional period between the Stone Age and the Bronze Age.

The Chalcolithic or Copper Age is the transitional period between the Neolithic and the Bronze Age. It is taken to begin around the mid-5th millennium BC, and ends with the beginning of the Bronze Age proper, in the late 4th to 3rd millennium BC, depending on the region.

Chalcolithic Age in India: c. 3000 BCE – 700 BCE
Bronze Age in India: c. 2600 BCE – 1900 BCE (during the Indus Valley Civilization)