Step 1: Definition.
National income is usually measured as GDP at market prices or related aggregates—i.e., ek desh mein ek saal ke dauran bane/supply hue sabhi final goods & services ki arthik keemat.
Step 2: Eliminate distractors.
(A) Family income: Micro level concept.
(B) Income of public enterprises: Sirf ek sector.
(C) Government income: Fiscal receipts only.
(D) Correct — poori economy ke final output ki money value.
Step 3: Conclusion.
Isliye national income ka sahi arth option (D) hai.