Answer:
d. changes in the supply of and/or demand for dollars in the global currency market.
Explanation:
Floating exchange rate can be defined as a system in the  macro economics or in economic policy where mechanism of the currency price of any country or nation can be determined by the forex market which is based on the supply and the demands relative to some other country's currencies.
In result of the foreign exchange values, the currency value of one country fluctuates.
Thus in the context, the value of dollar of United States changes depending on the changes or exchanges of dollar in the global market of currency.
