The correct answer is:
a. An increase in U.S. net capital outflow increases the supply of dollars and the dollar depreciates.
Explanation: When net capital outflow increases, it means that Americans are investing more in foreign assets or spending more abroad. This leads to a higher supply of dollars in the foreign exchange market, as individuals and businesses convert dollars to foreign currencies. An increased supply of dollars typically leads to a depreciation of the dollar's value relative to other currencies.