Final answer:
The most unusual aspect of the structure of the poem by Robert Louis Stevenson is sentences extending downward from one stanza into the next.
Step-by-step explanation:
The most unusual aspect of the structure of the poem by Robert Louis Stevenson (RLS) is option A) sentences extending downward from one stanza into the next. This technique, known as enjambment, is used to create a sense of fluidity and continuity in the poem. By not breaking the lines at the end of each stanza, the poem flows smoothly, merging ideas and concepts across stanzas.