Which of these was known in medieval British history as the founder and first king of Britain?

Brutus, or Brute of Troy, is a legendary descendant of the Trojan hero Aeneas, known in medieval British history as the eponymous founder and first king of Britain. This legend first appears in the "Historia Brittonum", an anonymous 9th-century historical compilation to which commentary was added by Nennius, but is best known from the account given by the 12th-century chronicler Geoffrey of Monmouth in his "Historia Regum Britanniae".

The "Historia Brittonum" states that "The island of Britain derives its name from Brutus, a Roman consul" who conquered Spain. This is ultimately derived from Isidore of Seville's popular 7th-century work "Etymologiae", in which it was speculated that Britain was named after the Roman general Decimus Junius Brutus Callaicus, who pacified Further Spain in 138 BC.