Answer:
C. Gatsby shows Nick a medal from Montenegro
Step-by-step explanation:
Nick went for a lunch with Gatsby wherein the latter recounts his story on drive, which is hard for Nick to believe.
To substantiate his narration and to take Nick into confidence, Gatsby hands over to Nick, various articles such as his old university days photograph, the medal he won at Montenegro.
The latter i.e the medal convinces Nick that Gatsby might have told the truth about his background, in the story he just narrated.