The most accurate prediction of the consequences of investors doubting the federal government's ability to repay the national debt is:
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 doubt the government's ability to repay its debt, they may be less inclined to buy government securities, which decreases demand. This reduced demand typically leads to higher interest rates, as the government would need to offer more attractive rates to entice buyers. Higher interest rates subsequently increase the cost of borrowing for the government when it issues new debt.