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

Price of one country's currency expressed in terms of another country's currency

Number of foreign dollars that can be purchased for every one U.S. dollar paid

Price that must be paid to obtain a good or service from another country

Price of a country's currency expressed in terms of that country's currency unit

Number of units of a currency that were originally required to obtain one euro when a country adopted the euro as its official currency