Answer:
A. Purchasing power parity
Explanation:
Purchasing power parity is a techniques that is used to determine the relative value or the exchange rates of currencies.
Eileen is using the purchasing power parity because she is comparing the cost effectiveness of buying a particular product in different countries using the dollar. The exchange rates of the currency of country X and country Y against will determine which country she will buy from.
In a nutshell, Purchasing power parity is a measurement of two currencies by taking the cost of living and inflation differences into account.