The national income of a country for a given period is equal to the

• (a) Correct. In standard macroeconomics, national income is the total value of goods and services produced by the nationals/residents of a country during a given period (more precisely, net factor income concept linked to nationals).
• (b) Incorrect. Consumption + investment is only part of aggregate expenditure; in an open economy/government sector, other components also matter.
• (c) Incorrect. Personal income is different from national income because it excludes some earned income and includes some transfer payments.
• (d) Incorrect. This describes domestic product/GDP at market prices more closely; national income is not simply the money value of final goods and services produced within the country, because the nationality/residence factor matters.

Revised view: My earlier answer was factually imprecise. UPSC is distinguishing national from domestic income; hence (a) is the best answer.