While ice skating, alice slipped and injured her knee. what is one way alice might have prevented this injury

It depends really. She might not have been concentrating enough and lost her footing in which case she needs to pay better attention to what she is doing. She could wear protective clothing on her knees and other places like her elbows just to be safe and as a preventative measure. She could also check her skates before going on the ice and make sure she is wearing them properly and they are sharpened and not dull.