Final answer:
The true statement about supply-side economics is that high marginal tax rates reduce the incentive to save, which is counterproductive for economic growth.
Step-by-step explanation:
The true statement of supply-side economics is that high marginal tax rates reduce the incentive to save. Supply-side economics suggests that economic growth can be most effectively created by lowering barriers to production, such as taxes. Lowering tax rates on businesses and individuals is thought to stimulate investment by making it more attractive to save and invest, owing to the potential for higher after-tax returns. Conversely, high tax rates are believed to discourage such investment behavior. Additionally, some supply-siders argue that lower tax rates could result in increased income and even potentially higher tax revenues despite lower rates because of increased economic activity.