The monster slain by Beowulf was Grendel. The third option is correct.
Beowulf is an Old English epic poem made up of of 3,182 alliterative lines. The story is set in Scandinavia. Beowulf is a hero of the Geats, who will help Hrothgar, the king of the Danes whose mead hall in Heorot has suffered attacks by a monster known as Grendel. Grendel is one of the poem's three antagonists. He is feared by all in Heorot but Beowulf.
