An economist proposes that the best remedy for declining productivity is to lower taxes, which would increase investment and ultimately increase productivity. this approach is called

This approach is called "supply-side economics".

Explanation:

Supply-side economics expect that there is improvement in economic growth by lowering tax rates because it provides individuals to work, save, and invest more. Thus in supply-side economics, consumers will have more profit when there is supply of goods is greater at lower prices.