The immediate cause of the attack by Nawab Siraj-ud-daullah on Calcutta in 1756 was the refusal of the English to demolish the fortification of Calcutta. This meant that the British were not complying with the Nawab`s order to dismantle the fortifications around the city.

Option 1, the refusal to pay trade tax, is not the correct answer because there is no mention of trade tax being the specific cause for the attack.

Option 2, the English conspiring against the Nawab, is not the immediate cause of the attack. While there were tensions between the English and the Nawab, the specific act of conspiracy was not the trigger for the attack.

Option 3, Siraj-ud-daullah wanting to drive out the English from Bengal, is not the immediate cause of the attack. While the Nawab did have disputes with the English and wanted to assert control over Bengal, this is not the specific reason for the attack.

Therefore, option 4, the refusal of the English to demolish the fortification of Calcutta, is the correct answer as it directly led to the attack by the Nawab.