The scale of a map tells us how much the size of the map feature on the paper corresponds to the actual size of that feature on the ground. In other words, it tells us how much the map has been reduced compared to the actual size of the area being mapped.
In the first statement, "one centimeter on the map represents half a kilometer on the ground", it means that every 1 centimeter on the map represents a distance of 0.5 kilometers on the ground.
In the second statement, "One centimeter on the map represents one kilometer on the ground", it means that every 1 centimeter on the map represents a distance of 1 kilometer on the ground.
In the third statement, "One centimeter on the map represents five kilometers on the ground", it means that every 1 centimeter on the map represents a distance of 5 kilometers on the ground.
In the fourth statement, "One centimeter on the map represents fifteen kilometers on the ground", it means that every 1 centimeter on the map represents a distance of 15 kilometers on the ground.