Answer:
The words that most effectively depict the idea that the flat tire will not be fixed easily are "jammed, suggesting the lug nut will remain stuck no matter what John does"
Step-by-step explanation:
The excerpt is talking about changing a flat tire and the difficulties that John has to lose a lug nut, then the adjective that is needed to complete the sentence has to be appropriate to objects, since relentless, unyielding and determined are more associated to human qualities jammed is the best option to describe something mechanic.