Answer: Buy U.S. dollars.
Explanation:
As the currency market is also controlled by the laws of supply and demand where an increase in demand increases price and a decrease in supply does the same, should the US want to increase the value of the dollar, they should buy more US dollars.
This action would increase the demand for the dollar while reducing the amount of dollars in the market. This will invariably lead to an increase in the price of the dollar all else equal.