The answer is all of these.
The Duke is the speaker of the poem where he is entertaining an emissary who has come to negotiate the Duke's marriage. As he shows the visitor through his palace, they came across the portrait of the late Duchess. The Duke then began to reminisce about the portrait sessions and then about the Duchess herself. As his monologue continues, the reader realizes that it was the Duke who caused the untimely demise of the previous Duchess.
The character’s actions are steady and reliable
A man (or a human to be politically correct). In Greek mythology, a monster in the city of Thebes called the Sphinx would ask this question to any passerby and if they couldn't answer it correctly, the Sphinx would eat him/her. In the morning, the man is a baby and he crawls on fours. In the afternoon, a man is grown and walks on two feet. In the evening, the man is old and requires a cane to walk. The man who solved this riddle was Oedipus who, upon solving it, caused the Sphinx to fling herself off a cliff and commit suicide.
