In <em>The Solitary Reaper</em>, the speakers expresses his feelings about the reaper's song. The reaper was a lady, apparently with the most beautiful voice the speaker had ever heard. He is overtaken by the beauty of her tune and goes on wondering about what the reaper could be singing about. He ends the poem stating that, regardless of the meaning of her song, he carries the beauty of it in his heart even when he can't hear her anymore.
Taking into consideration that one of the most prominent features of Romanticism is the focus on emotions, and that this poem is all about the speaker's emotions and imagination regarding the reaper's singing, we can conclude that this is the most prominent Romantic characteristic in this poem.
Therefore, your best answer is <em>emotion.
</em><u>No excerpt was provided for a more specific element.</u><em> </em>