“My Last Duchess” is actually a poem that was written by Robert Browning. And based on this poem, the reason as to why run-on lines were being used by the author is to express long, complex ideas and emotions. The best answer for this is the first option. Hope this helps.
In “My Last Duchess” by Robert Browning, why did the poet use run-on lines? to express long, complex ideas and emotions to show that the duke prevents the agent from speaking to convey that the duke belongs to the upper class to give the duke’s words the natural rhythm of conversation