Kate takes the elevator from the ground floor (which is labeled 0) to a floor three levels below the ground floor. which number represents the floor where she gets off the elevator?

The number should be -3. That is because she goes from 0 to three floors bellow, meaning it's 0-3= -3. The first floor bellow would be -1, the second would be -2, and the third one is -3. It would be different if you counted it as number 1 as they do in some countries, but here it's -3.