Which of the Following Is the Best Definition of Exchange

Which of the following is the best definition of exchange rate.

A) The market where one country's currency is traded for another's.
B) International bonds issued in a single country, usually denominated in that country's currency.
C) Federal Crown corporation that promotes Canadian exports by making loans to foreign purchasers.
D) The idea that the exchange rate adjusts to keep purchasing power constant among currencies.
E) The price of one country's currency expressed in another country's currency.