Explanation:

- Base Year: The base year is the year against which economic growth or inflation is measured. It's used as a benchmark to calculate real GDP, ensuring consistency by adjusting for inflation.

- Correct Answer: Base Year.

- Fiscal Year: This is a 12-month period used for accounting purposes. It is not restricted to the calendar year and varies between different organizations or governments used for budget and tax purposes.

- Financial Year: Similar to a fiscal year, it is the 12-month period in which a business organizes its financial reporting. It may vary from the calendar year but is not used to calculate real GDP.

- Common Year: A common year is a calendar year with 365 days as opposed to a leap year with 366 days, and it's not related to GDP calculations.