I hope I understood the question correctly.This was a tricky question. I believe that "their" would be your correct answer because . .
→ The word "Their" mean's: to have possession. In the statement above. It clearly state's that everyone has "their" individual car, and they parked "their" car in lot B.
→ The other's word's that sound like "their" are (there and they're). So the word "there" mean's at a location. For example: "Jurgen was there at the bowling alley." And the word "they're" mean's "they are". So this would not have gone in that sentence.
