The correct answer is C. The Roman pull-out from Britain.
Indeed, the soldier Roman soldier Flavius Claudius Constantinus decided to take the title of Western Roman Emperor in 407 A.D. (5th century), adopting the imperial name of Constantine III. He failed, however, to protect Roman Britain from Anglo-Saxon invasions and raids and his rule was ultimately rejected by the Roman-Britons who decided to arm themselves. and organize their own protection. They also failed to protect their territory from these invasions and were conquered by the Anglo-Saxons, who ended up dividing the territory into seven Saxon Kingdoms, the Heptarchy.
