The correct answer is option 'A': Real exchange rate.

Explanation:
The ratio of foreign rates to domestic rates measured in the same currency is known as the real exchange rate. Let's break down this explanation into the following sections:

1. Nominal Exchange Rate:
The nominal exchange rate refers to the rate at which one currency can be exchanged for another. It represents the value of one currency in terms of another currency. For example, if the exchange rate between the US dollar and the Euro is 1.2, it means that 1 US dollar can be exchanged for 1.2 Euros.

2. Foreign Rates and Domestic Rates:
Foreign rates and domestic rates refer to the interest rates in different countries. The foreign rate represents the interest rate in a foreign country, while the domestic rate represents the interest rate in the home country.

3. Measured in the Same Currency:
To compare interest rates between countries, it is important to measure them in the same currency. This allows for a direct comparison of the rates without the influence of currency exchange.

4. Real Exchange Rate:
The real exchange rate takes into account the nominal exchange rate and adjusts it for the relative price levels between two countries. It is calculated by dividing the nominal exchange rate by the ratio of the foreign price level to the domestic price level.

The formula for calculating the real exchange rate is as follows:
Real Exchange Rate = Nominal Exchange Rate x (Foreign Price Level / Domestic Price Level)

The real exchange rate reflects the purchasing power of one currency relative to another currency. It indicates how much of one country's goods and services can be exchanged for a given amount of another country's goods and services.

In summary, the ratio of foreign rates to domestic rates measured in the same currency is known as the real exchange rate. It takes into account the nominal exchange rate and adjusts it for the relative price levels between countries.