The answer is D. an exchange of currencies happens when you "trade" one currency for another, which can also be thought of buying one currency in the form of another currency.
So for example, if you were going to exchange the US Dollar for Mexican Pesos, the exchange rate is 1 USD to 17 MXN. Therefore, to get 17 MXN, you need to pay 1 USD.
Does that make sense?
D. Price ceiling
This is a government regulation that establishes a maximum price for a specific thing.