Answer: Appreciate, nominal
Explanation: If a nation's currency doubles in value, it is said to appreciate, thereby reflecting a change in the nominal exchange rate. An increase in a nation's currency signifies an appreciation of its currency. When the currency appreciates, it affects the exchange rate at which the currency can be traded.
Option b is not correct because when a currency appreciates, it does not necessarily affect the real exchange rate. Real exchange rate means the rate at which goods and services of a country can be traded for goods and services of another country.