Final answer:
Sir Gawain is shown to be a knight who believes in social justice through his brave and valorous actions, such as facing the Green Knight and defeating a dragon, which are in line with the chivalric code.
Step-by-step explanation:
The excerpt from the story reveals that Sir Gawain is a knight who believes in social justice. The passage describes how Sir Gawain, having defeated a dragon, approaches a shield which is identified with the breaking of an enchantment. His brave and valorous action in facing the after-effects of dragon combat demonstrates his courage and adherence to knighthood's principles, which often include fighting for justice and protecting the innocent.
Therefore, the correct answer to the question - 'Which of the following events reveals that Sir Gawain believes in social justice?' - is 'a) Meeting the Green Knight'. The act of facing a knight known for his mystical powers and the subsequent adventures highlight Gawain's dedication to his knightly duties, which are inherently tied to the chivalric code, including social justice.