Final answer:
Banquo says the line "Why do you start and seem to fear things that do sound so fair?" to Macbeth after the witches' prophecies, questioning why Macbeth is frightened by such seemingly good news.
Step-by-step explanation:
The line "Why do you start and seem to fear things that do sound so fair?" is spoken by Banquo to Macbeth. This exchange occurs after the three witches have delivered their prophecies, heralding Macbeth as the Thane of Glamis, Thane of Cawdor, and the future king. Banquo is puzzled by Macbeth's startled reaction to prophecies that seem to be favorable and questions why Macbeth appears to be frightened by them.