Which One of the Following Is the Best Universal Definition

Which one of the following is the best universal definition of an exchange rate?

A) number of foreign dollars that can be sold for every one US dollar paid
B) the number of units of a currency that were originally required to obtain one euro when the country adopted the euro as their official currency
C) the price,which includes the government fee,to purchase a country's currency
D) the price of a country's currency expressed in terms of that country's currency unit
E) price of one country's currency expressed in terms of another country's currency