Final answer:
Sir Gawain keeps the green sash as a secret weapon against the Green Knight in the hopes that its magical properties will protect him from harm, thus compromising his honor.
Step-by-step explanation:
In the poem "Sir Gawain and the Green Knight", Sir Gawain keeps the green sash given to him by the lady not as a token of their love, a symbol of her favor, or a memento of his adventure. Instead, the correct option is d) As a secret weapon against the Green Knight. The lady tells Sir Gawain that the green sash possesses magical properties that will protect him from harm. Since Sir Gawain is anticipating a lethal encounter with the Green Knight, he accepts the sash in hope that it will save his life, thus acting against his code of chivalry that values honesty and honor.
His acceptance of this sash becomes a pivotal point in the story, as it symbolizes his human frailty and fear of death, which leads him to compromise his honor as a knight of the Round Table.