In "Dover Beach," the "distant northern sea" is contrasted to: -Ionian -Sea of Faith -girdle -tremulous cadence
It is always important with these multiple choice questions to read the quote in context of the lines around it and then to use a process of elimination to go through the various options one by one, deleting those that you know it definitely isn't before finally selecting the right one. Reading this poem carefully, you can see that the sound of the waves of this "distant northern sea" then prompts the speaker to compare this sea to the metaphorical "Sea of Faith." This metaphor stands for the beliefs that the speaker fears are disappearing from the world. Just as the tide of the sea withdraws, so the speaker fears that the tide of the Sea of Faith is withdrawing, indicating that something protective and powerful is leaving the world, exposing it to danger:
The Sea of Faith
Was once, too, at the full, and round earth's shore
Lay like the folds of a bright girdle furled.
But now I only hear
Its melnacholy, long, withdrawing roar...
So the correct answer is that the "sea" is compared to the "Sea of Faith," the poignant metaphor that Arnold uses to discuss the decline of faith in his context.