Ответ:
2. Ivan and Anton's bike
3. Lena's bike
4. Lena and Vera's bike
5. Vera and Lena's bike
6. This boy's bike
7. These boys' bike
8. These girls' bike
9. This girl's bike
10. This child's bike
11. These children's bike
12. These twins' bike
13. This athlete's bike (один из вариантов перевода слова "спортсмен")
14. These athletes' bike