The partition of Bengal in 1905 was realized during the Viceroy-ship of:

Option 1 -> Lord Mountbatten was the last Viceroy of India (1947) who oversaw the partition of India and Pakistan, not the 1905 Bengal partition.

Option 2 -> Lord Dalhousie served as Governor-General from 1848-1856, which was decades before the 1905 partition of Bengal.

Option 3 -> Lord Irwin was Viceroy from 1926-1931, more than two decades after the 1905 Bengal partition had already occurred.

Option 4 -> Lord Curzon served as Viceroy from 1899-1905 and implemented the partition of Bengal in 1905, ostensibly for administrative efficiency.

Hence, Option 4: Lord Curzon -> The partition of Bengal was carried out during Lord Curzon's viceroyship in 1905, dividing the Bengal province into East Bengal & Assam and West Bengal. This administrative decision had significant political ramifications and sparked widespread protests, eventually leading to its annulment in 1911 -> correct