Explanation:

- The Harappans had a vast trade network and resource system. They excelled in metallurgy.

- Copper was a vital metal for Harappans. It was extensively used for making tools, ornaments, and vessels.

- Copper was obtained from the present-day Rajasthan region, particularly from areas like Khetri.

- Other metals like iron and aluminum were not utilized by Harappans as their metallurgical techniques were not advanced enough at that time.

- Calcium is not a metal; it is an essential mineral in various forms but not directly related to metallurgy.

Correct Answer: Option 3 - Copper