A challenge to fiscal sustainability in Tamil Nadu is:

Increasing debt burden
Rising pension liabilities
Declining tax revenue
All of the above

The correct answer is: All of the above.

Increasing debt burden: Tamil Nadu’s debt burden has been increasing in recent years. As of 2020, the state’s debt was over ₹6 lakh crore, which is equivalent to about 30% of its gross state domestic product (GSDP). This high debt burden is a major challenge to the state’s fiscal sustainability.

Rising pension liabilities: Tamil Nadu also has a large pension liability. As of 2020, the state’s pension liability was over ₹2 lakh crore. This pension liability is expected to grow in the coming years, as the state’s population ages. This rising pension liability is another major challenge to the state’s fiscal sustainability.

Declining tax revenue: Tamil Nadu’s tax revenue has been declining in recent years. This decline is due to a number of factors, including the slowdown in the economy, the implementation of the Goods and Services Tax (GST), and the reduction in the state’s share of central taxes. This decline in tax revenue is a major challenge to the state’s fiscal sustainability.

In conclusion, all of the options mentioned in the question are challenges to fiscal sustainability in Tamil Nadu. These challenges need to be addressed in order to ensure the state’s long-term fiscal health.