Final answer:
Hrothgar says that past fighters who swore to protect Herot were killed by the monster Grendel. This is part of the story of 'Beowulf', where Herot is a mead-hall under attack.
Step-by-step explanation:
The original question posed is: "What does Hrothgar say has happened to past fighters who swore to protect Herot?" The answer is b) "They were killed by Grendel." In Beowulf, Hrothgar, the king of the Danes, laments that many brave warriors who vowed to defend the great mead-hall, Herot, were tragically slain by the monstrous Grendel, who plagued the hall and terrorized its inhabitants.