Final answer:
The correct answer is d. Trade deficit. The ratio of public debt to national income is not directly affected by the trade deficit, but is influenced by interest rate, economic growth, and primary budget surplus.
Step-by-step explanation:
The correct answer is d. Trade deficit. The ratio of public debt to national income is not directly affected by the trade deficit. The ratio is influenced by factors such as interest rate, economic growth, and primary budget surplus. When the interest rate is high, it can increase the cost of borrowing for the government, leading to a higher debt ratio. Economic growth can increase national income, which can help lower the debt ratio. A primary budget surplus means that the government is generating more revenue than it is spending, which can lower the debt ratio.