The correct alternatives are:
A) "The amount of one currency that can be traded for a unit of another currency is the exchange rate". That is precisely correct. For example, to buy 1 US Dollar, you need 0.89 Euros. (11/28/2018)
B) "Many exchange rates float, or constantly change". That is true. If you're planning to buy units of another current you must be attentive to the ups and downs of their value, since it's not stationery and depends on lots of factors.
C) "It is best to travel abroad when the exchange rate gets you more units of a foreign currency". It goes without saying that it's best to travel with as much money as possible. So, traveling when the exchange rate gets you more units of a foreign currency is the perfect scenario. Using our previous example, it would be better to travel to the United States when you can buy 1 US Dollar for 0.80 Euros rather than getting it for 0.89 Euros.
