Thematically, the situation that would fit the story of Beowulf is B. Hrothgar asks Beowulf to join him in a raid on a rival kingdom.
The reason is that, in both parts the poem, Beowulf is presented as a hero led by altruism and ancient germanic laws of loyalty to the people and the ruler of the land. If the king asked him to raid a rival kingdom, Beowulf would see no reason to disobey him.
Futhermore, option A is unthinkable: the king shows no sign throughout the poem of wanting Grendel to devour his people. Option C is impossible: Beowulf dies in the second part of the poem after fighting the dragon. Option D is also impossible: Beowulf presents Grendel's head before the king after slaying Grendel's mother.
