Aballerina wants to improve her leg extension. what type of activity would be the most appropriate to focus on during training to accomplish this goal?

My guess would be stretching in the leg she wants to improve. I have done this many times and stretching would be the best way. 
You can go into a split and then lean as far forward as you can.
Another one is to when in the split grab your toes and pull up,stretching near the actual foot which then gives you more of a chance to lift your leg higher. 

True. the more you know, the better of you will be.