Final answer:
In the story 'Sir Gawain and the Green Knight', Sir Gawain is the one who accepts the challenge to fight the Green Knight to protect King Arthur and uphold the honor of Camelot.
Step-by-step explanation:
In the context of the story where a giant green knight challenges King Arthur's knights, the person who finally speaks up to fight the knight is b) Gawain. This story, known as Sir Gawain and the Green Knight is a classic of medieval literature, and Sir Gawain is King Arthur nephew and one of the knights of the Round Table. Gawain accepts the challenge as a way to protect his lord, King Arthur, and to uphold the honor of Camelot.