Final answer:
The main subject of the speaker's monologue in Robert Browning's "My Last Duchess" is the Duke's deceased wife, the Duchess, whose portrait provokes a display of possessiveness and control from the Duke.
Step-by-step explanation:
The central subject of the speaker's monologue in Browning's "My Last Duchess" is the Duke's late wife, the Duchess, whose portrait hangs on the wall. This dramatic monologue reveals the Duke's perspective on his former spouse, providing insight into his character. The Duke speaks of his late Duchess as an object of beauty, yet his jealousy and control over her are evident. The poem explores themes of power, possessiveness, and the objectification of women.