To reduce a national deficit, governments canincrease taxationto generate more revenue. This measure, combined with controlled spending, helps reduce the shortfall between revenues and expenditures.

B. Increase government spending: This would increase the deficit further unless matched by revenue increases.

C. Decrease taxation: This would reduce revenue and worsen the deficit.

D. Increase interest rates: This impacts monetary policy and borrowing costs but does not directly reduce a fiscal deficit.