Option 1 states that the English levied heavy duties on the goods entering Calcutta as the cause of the Battle of Plassey. This option suggests that the English imposing heavy taxes led to the conflict. However, this option is not accurate as it does not align with the historical events surrounding the battle.

Option 2 argues that Siraj-ud-Daulah suspected that the English favored his rivals, which caused the Battle of Plassey. This option implies that suspicion and mistrust between Siraj-ud-Daulah and the English were the underlying cause of the battle. While this option is plausible, it is not the correct answer given in the question.

Option 3 correctly states that the English began to fortify Calcutta as the cause of the Battle of Plassey. This option explains that the English strengthening their defenses in Calcutta incited Siraj-ud-Daulah to take action against them. This is in alignment with historical accounts and is indeed the correct answer provided in the question.

Option 4 suggests that Siraj-ud-Daulah was instigated by the French, causing the Battle of Plassey. However, this option does not have any evidence or historical backing, making it an incorrect answer