The correct answer is Punjab National Bank.
In older general knowledge references, Punjab National Bank (PNB) was cited as the largest nationalised bank in India in terms of the number of branches it operated.
Note that rankings among nationalised banks have shifted over the years due to mergers, such as the merger of several public sector banks in 2019 and 2020, so this answer reflects the traditional or legacy fact typically asked in exams rather than the current ranking.