Which statement best explains the impact the in medias res structure has on the epic Beowulf

To simplify the other answer, it's The antagonist Grendel is introduced very quickly.

The antagonist Grendel is introduced almost immediately.

In Media Res means that the narrative begins in the middle of the action (usually giving an insight of the events leading to the action through flashbacks or during a dialogue).

In Beowulf’s case, the story begins right away with the antagonist, Grendel, attacking the Heorot, a place where men gathered to eat, drink and listens to songs of the bards, songs which themes regarded God’s creations and these themes were what triggered Grendel’s anger.

The presentation of the antagonist at the beginning of the narration is a stylistic conventionfor Epic Poetry.