Perhaps the eye of a scrutinizing observer might have discovered a barely perceptible fissure, which extending from the roof of the building in front, made its way down the wall in a zigzag direction, until it became lost in the sullen waters of the tarn.

how does the description of the house hint at the ending of edgar allan poe’s “the fall of the house of usher”?
a: the crack indicates that the house is vulnerable to collapse
b: the observer is the narrator, who sees danger in the roof line
c: the masonry is broken and decayed, like the narrator’s mind
d: the water reference indicates that death will come via water

B: The observer is the narrator, who sees danger in the roof line

Explanation:

The crack is "barely perceptible", still it spreads for a long distance throughout the house until it disappears in the water. The focus here, though, is not on the masonry, nor on the observer and for sure, it has nothing do with the water itself, but with the idea that although the crack is barely perceptible, it shows an imminent danger since it spreads throughout the whole foundation of the House. The literary device used here is the literary technique of foreshadowing that predicts the sad end of Usher and the literal fall of the House of Usher.