The correct option is 1971 Census.
Explanation
Under Article 82 of the Constitution, the Parliament enacts a Delimitation Act after every census to establish a Delimitation Commission. This body determines the allocation of seats in the Lok Sabha to the states and the division of each state into territorial constituencies based on population data.
Detailed Analysis:
- 42nd Amendment Act, 1976: This amendment froze the allocation of seats in the Lok Sabha to the states and the division of each state into territorial constituencies till the year 2000 at the 1971 level. This measure was taken to ensure that states implementing family planning and population control measures were not penalized politically by a reduction in their representation relative to states with higher population growth.
- 84th Amendment Act, 2001: This amendment extended the ban on the readjustment of the total number of seats in the Lok Sabha for another 25 years (i.e., up to the year 2026). Consequently, the number of Lok Sabha seats remains based on the 1971 Census.
- Distinction regarding 2001 Census: While the total number of seats is frozen based on the 1971 Census, the 87th Amendment Act (2003) provided for the delimitation (readjustment of boundaries) of constituencies within a state based on the 2001 Census figures, without altering the total number of seats allocated to that state.
Key Takeaway:
The total number of seats in the Lok Sabha is frozen based on the 1971 Census, whereas the internal boundaries of constituencies are readjusted based on the 2001 Census.