Final answer:
In Poe's 'Haunted Palace,' the lines in question serve as a metaphor for depression, which is typical of Poe's themes of darkness and despair.
Step-by-step explanation:
In Edgar Allan Poe's "Haunted Palace," the lines "But evil things, in robes of sorrow, Assailed the monarch's high estate;" utilize a poetic device known as a metaphor. The correct answer to the student's question is D. A metaphor for depression. This is because the lines describe the figurative 'assault' on the 'monarch's high estate' which, within the context of the poem, can be understood as a representation of a mind once noble and high being overtaken by sadness and depression. Poe's work often delves into themes of darkness and despair, and this metaphor is consistent with his common motifs of psychological distress and decay.