The correct response to the question about the consequences of investors doubting the federal government's ability to repay the national debt would be:
Fewer investors would be purchasing the securities from the Treasury, resulting in an increase in interest rates, and a further increase in the cost of borrowing for the national debt.
When investors lose confidence in the government's ability to repay its debt, they may demand higher interest rates to compensate for the increased risk, leading to higher borrowing costs for the government.