Answer:
C: The sixth-grade students liked volunteering their time to read to the eldery people in the hospital. It made the volunteers happy.
Step-by-step explanation:
The sentence It made the volunteers happy. corrects the pronoun error in the passage because it makes clear who becomes happy with the action of "volunteering their time to read to the eldery people." In the options A and B, it´s not clear: It made who happy? In D: Who made who happy? In E: Reading made who happy? (the volunteers or the elderly people ?)