Which one of the following was the cause of the Battle of Plassey ?

The Battle of Plassey (1757) was primarily caused by the East India Company's refusal to halt military preparations and the fortification of Calcutta. Siraj-ud-Daulah, the Nawab of Bengal, was alarmed by the Company's unauthorized construction of fortifications at Fort William, which they justified as a defense against the French during the Seven Years' War [t4][t6]. The Nawab demanded the removal of these fortifications, but the British refused to comply [t4][t7]. This defiance, coupled with the Company's abuse of trade privileges (dastaks) and their harboring of political fugitives like Krishna Das, led the Nawab to seize the English factory at Kasimbazar and capture Calcutta in 1756 [c4][t6]. While the British did levy duties on Indian goods entering Calcutta [c4], the immediate casus belli for the Nawab's military action—which culminated in the Battle of Plassey—was the unauthorized fortification and the challenge to his sovereignty.

Sources

1. [1] https://www.nam.ac.uk/explore/battle-plassey
2. [2] Modern India ,Bipin Chandra, History class XII (NCERT 1982 ed.)[Old NCERT] > Chapter 4: The British Conquest of India > British Occupation of Bengal > p. 66