Which Anglo-Saxon kingdom of Great Britain was unified by Æthelstan in 927?

king athelstan was elected king of mercia, then wessex in the same year, although his father king Edward the elder was king of both kingdoms most of his adult life (his sister aethelflaed ruled mercia in partnership with her brother for sometime, Edward was the king of both. so it doesn't really count that aethelstan unified wessex. he eventually untied most of modern day England in too one kingdom and was generally recognised as being the liege lord of the other kingdoms on the island of Britain