Final answer:
d. a damsel in distress. The primary plot element missing from the excerpt from 'Sir Gawain and the Green Knight' is a damsel in distress.
Step-by-step explanation:
The primary plot element missing from the excerpt from 'Sir Gawain and the Green Knight' is d. a damsel in distress. This plot element is a common characteristic of medieval romances, where the hero typically rescues a damsel in distress. However, in the excerpt from 'Sir Gawain and the Green Knight,' there is no mention of a damsel in distress or any similar situation.