The revenue deficit is a crucial component of a government's fiscal deficit and is calculated to measure the shortfall of the current receipts over current expenditure.  

  2.Here's the reason why the formula (2) is used to calculate the revenue deficit:

(2) Revenue expenditure - Revenue receipts: This formula signifies that the revenue deficit is the difference between the government's total revenue expenditure and its total revenue receipts. In other words, it represents the amount by which the government's current expenditure exceeds its current revenue.

Options (1), (3), and (4) do not represent the standard formula for calculating the revenue deficit. 

Option (1) involves the total expenditure, not just the revenue expenditure. Option (3) involves interest payments, which are not directly related to the concept of the revenue deficit. Option (4) involves the fiscal deficit, which is a broader concept compared to the revenue deficit.

Hence, the correct answer is (2) Revenue expenditure - Revenue receipts.