Explanation:

- Muhammad bin Tughlaq introduced a copper coin called “token currency” in place of the silver coin during his reign.

- The correct name of this copper coin is “Tanka”.

- Let's look at the options:

- Rupak: This was not a coin introduced by Muhammad bin Tughlaq.

- Rupee: The rupee was introduced later by Sher Shah Suri, not Tughlaq.

- Jittal: It was a small denomination coin, but not the copper token introduced by Muhammad bin Tughlaq.

- Tanka : This was the coin introduced by Muhammad bin Tughlaq as a copper currency to replace the silver tanka.