The correct answer is option 1, interest payments. Non-plan expenditure refers to the expenses incurred by the government that are not part of a planned program or development initiative. Interest payments are a significant component of non-plan expenditure as the government has to pay interest on its borrowings, including loans from international institutions and market borrowings. These interest payments include both domestic and external debt obligations. The government`s interest payments are substantial due to the size of its debt and can consume a significant portion of its budget. While options 2, 3, and 4 (subsidies, defence, and salaries and wages) are also important components of non-plan expenditure, interest payments typically account for a larger share and hence can be considered the biggest head of non-plan expenditure for the Government of India.