Agree white rock. Leghorns have yellow legs and single combs, but white earlobes. White Rocks have yellow legs, single combs and red earlobes. LEghorms are generally pretty trim in build, whereas a White Rock has a larger build, more muscle bc it's dual purpose.
JG do not have yellow legs (even the white ones), and body structure is different.
Cornish would be larger (rotund) and shorter, with thicker legs at this age.