Explanation:

- The Mughal firman of free trade in Bengal was a decree issued by the Mughal emperor. It granted the English East India Company the right to trade freely without paying the usual duties.

- Option 1: 1717

- In 1717, the Mughal Emperor Farrukhsiyar issued the firman allowing the British East India Company duty-free trade in Bengal.

- Option 2: 1756

- In 1756, there was no specific firman. However, this period was marked by tensions leading to the Battle of Plassey in 1757.

- Option 3: 1650

- This year predates significant British trading rights in Bengal and does not correspond with any firman for free trade.

- Option 4: 1696

- This date is associated more with Dutch and French activities in India and not with an English trading firman in Bengal.