Weird , hope this helps!!!
Well, the very last two lines state quite explicitly that the word <em>sepulchre </em>means grave:
<em>In her sepulchre there by the sea-
</em><em>In her tomb by the sounding sea. </em>
Using "Tekton" we can guess that tectonic means something related to building, carpentry, or construction
A is the answer i believe