Answer and Explanation:
1. B: "...It made the volunteers happy." ( The pronoun them shows ambiguity. )
2. D: "...family...they...their." The word family can be singular or plural because it is a collective noun.
3. C: The bikes. The pronoun they refer to the bikes because only the bikes could have needed repair, not the door (singular) nor the hedges.
4. B: If people want to help their comunity, they should research opportunities. People is plural word and the corresponding pronouns are they and their .
5. D: them. ( the floor and the counter )
6. C: volunteering. The pronoun It corresponds to the action of volunteering.
7. B: them. The pronoun them has an unclear antecedent because it is difficult to know if it refers to 'the six-grade students' or to 'the eldery people'.
8. C: its. Refering to the town's beaches.
9. C: The use of they/them agreeing with somebody is possible only in colloquial English.
10. their. The student (he or she).