If Americans become less concerned with the future and save less at each real interest rate, this implies that the supply of savings decreases. When the supply of savings decreases, the equilibrium interest rates in the market will be affected.
In this scenario, as individuals save less, there is less capital available for investment. This would typically lead to higher real interest rates, as there is less money available for borrowing. With higher real interest rates, businesses are likely to invest less, as the cost of borrowing increases.
Therefore, the correct answer is:
d. real interest rates rise and investment falls.