Option 1 -> Incorrect sequence as factor income classification must come before NDPFC estimation.
Option 2 -> Incorrect sequence as NNPFC cannot be estimated before NDPFC.
Option 3 -> Correct sequence following the logical steps of income method.
Option 4 -> Incorrect sequence as factor income classification should precede NDPFC estimation.
Hence, Option 3: (A), (D), (B), (C) -> The correct sequence for estimation of National Income by income method is: First, identify and classify producing firms (A), then classify factor incomes into wages, rent, interest, and profits (D), next estimate Net Domestic Product at Factor Cost by summing all factor incomes (B), and finally estimate Net National Product at Factor Cost by adding Net Factor Income from Abroad to NDPFC (C). This logical flow ensures systematic calculation from identification to final national income estimation -> correct