Final answer:
Gloucester addresses King Lear in the lines, which highlight Gloucester's feeling of being deceived rather than defeated.
Step-by-step explanation:
The lines, "This is practice, Gloucester. By th' law of war thou wast not bound to answer An unknown opposite. Thou art not vanquish'd, But cozen'd and beguil'd," are spoken to King Lear in the play King Lear by William Shakespeare. This excerpt is not found in the play Macbeth, so the correct answer to who Gloucester is addressing is d) King Lear. The lines express that Gloucester believes he has been deceived, rather than truly defeated in conflict.