The correct answer is - kilometers.
Very often, the map scale is showing distances in both miles and kilometers. The reason for this that the majority of the countries in the world use the metric system, thus the kilometers are a must, but also there countries that do not use this system, thus they include the miles. In this way, the map scale can be used by people that both use and not use the metric system in their daily lives.
The map scale is a very important element on the maps. Its purpose is to enable the reader of the map to be able to measure the distance from one place to another, thus to be aware of the real distance in the real space.