In grendel, john gardner uses

first-person point of view to readers better understand beowulf’s feelings.
first-person point of view to readers see grendel’s side of a familiar story.
third-person point of view to give greater insight into all of the characters.
third-person point of view to give an impartial account of a well-known event.

First-person point of view to help readers see Grendel’s side of a familiar story.

Explanation:

John Gardner rewrote the old English poem of "Beowulf" but from the antagonist's perspective. His "Grendel" is narrated from the first person perspective of Grendel and gave his side of the story.

By giving the narrative voice to Grendel, Gardner gave a different view of the otherwise famous story of Beowulf. Through the perspective of Grendel, we came to know how he really feels in the story. He also shows us what it feels like to be a monster and treated badly like he was treated by the people. This first person point of view helps us readers to see Grendel's side of a story all too familiar.

the purpose of the heading in the passage is to identify who is telling the story.

this excerpt is part of the book sugar changed the world (2010) written by marc aronson and marina budhos. as the book has been written by two people, the heading "marc" indicates that this specific excerpt is being narrated by aronson. in the fragment, marc describes his grandfather's story after hearing it from his cousin. however, there are other excerpts where budhos is the one that tells the story of her family.

aronson and budhos, who are husband and wife in real life, decided to write sugar changed the world when they found out how sugar, which had an important role in slavery and economy, changed their ancestors' lives.