Решение:
Смотрим на предложения a-c и вставляем подходящие притяжательные местоимения.
- It's OK to take a phone, but she doesn't take hers.
- In some schools they take smartphones, but not mine.
- I bring my phone to class and the students bring theirs.
Ответ: 1. hers, 2. mine, 3. theirs.