What Does the Term Exchange Rate Refer To

What does the term exchange rate refer to?  

A)  to the total yearly amount of money changed from one country's currency to another country's currency 
B)  to the amount of a country's currency that can be traded for one ounce of gold 
C)  to the sum of net unilateral transfers 
D)  to the price of one country's currency in terms of another country's currency