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) Price of one country's currency expressed in terms of another country's currency
B) Number of foreign dollars that can be purchased for every one U.S. dollar paid
C) Price of a country's currency expressed in terms of that country's currency unit
D) Number of units of a currency that were originally required to obtain one euro when a country adopted the euro as its official currency
E) Price which must be paid to obtain a good or service from another country