That is a Brown Leghorn pullet. Possibly also a Welsummer - her earlobes don't seem to have much color either way, but her skinny body suggests Leghorn. If she develops white earlobes, she is a Leghorn; if they are a red, she is a Welsummer.
I'm pretty sure that's a light brown Leghorn pullet. The white earlobe isn't too prominent yet, but I think I can see it. Other contenders would be Welsummer, but she's kind of lean for that breed. She could possibly be a game hen, but the shape ins't quite right for that either.