To find the increase in the U.S. national debt from 1971 to 2021, we need to subtract the debt in 1971 from the debt in 2021.
The debt in 2021 is: \[ 2.9617 \times 10^{13} \]
The debt in 1971 is: \[ 3.98 \times 10^{11} \]
To perform the subtraction, it's helpful to express both values with the same exponent. We can rewrite \(3.98 \times 10^{11}\) with a base of \(10^{13}\):
\[ 3.98 \times 10^{11} = 0.0398 \times 10^{13} \]
Now we can subtract the two values:
\[ 2.9617 \times 10^{13} - 0.0398 \times 10^{13} = (2.9617 - 0.0398) \times 10^{13} \]
Calculating \(2.9617 - 0.0398\):
\[ 2.9617 - 0.0398 = 2.9219 \]
Thus, the increase in the national debt is:
\[ 2.9219 \times 10^{13} \]
Now, looking at the options provided, the correct answer is:
\[ \boxed{2.9219 \times 10^{13}} \]