The fortification of Calcutta by the British in 1756 was seen by the Nawab of Bengal, Siraj-ud-daulah, as an attack upon his sovereignty. This means that the Nawab perceived the actions of the British in fortifying Calcutta as a threat to his authority and control over the region.

Option 1 suggests that the fortification was related to the growth of large-scale British trade. However, there is no indication in the question that this was the motive for the fortification.

Option 3 suggests that the British were insecure in India, leading to the fortification. While it is true that the British faced security concerns in India, this was not the reason specifically mentioned in the question.

Option 4 suggests that the fortification represented British control over Bengal. This is partially true, as the fortification did signify British presence and influence in Bengal. However, the primary reason mentioned in the question was that it was seen as an attack on the Nawab`s sovereignty.

In summary, the correct option is option 2, as the fortification of Calcutta was regarded by the Nawab as an attack upon his sovereignty.