The correct choice is: b. apostrophe in "Marcs" and "uncles".
The sentence should be corrected to: "Marc's ambition is to buy his uncle's 1947 Dodge and restore it to its original condition."
- "Marc's" indicates possession (the ambition of Marc).
- "uncle's" indicates possession (the Dodge belongs to his uncle).
- "its" is correct as it is (no apostrophe is needed in that context as it shows possession).