Final answer:
In 'Sir Gawain and the Green Knight,' Sir Gawain keeping the magic girdle represents a deviation from the ideals of chivalry.
Step-by-step explanation:
In the story 'Sir Gawain and the Green Knight,' the event that represents a deviation from the ideals of chivalry is option B: Sir Gawain keeps the magic girdle.
Chivalry is a code of conduct that promotes knightly virtues, such as honesty, loyalty, and honor. However, Sir Gawain's decision to keep the magic girdle, which grants him protection, is a betrayal of these ideals. He fails to uphold the principle of honesty by concealing the truth from his host, the Green Knight, and thus violates the code of chivalry.
This deviation from chivalry serves as a test of Gawain's moral integrity and highlights the complexities of upholding noble values in challenging situations.