plzzzz help Which of the following corrects the pronoun error in this passage?

The sixth-grade students liked volunteering their time to read to the elderly people in the hospital. It made them happy.

The sixth-grade students liked volunteering his or her time to read to the elderly people in the hospital. It made them happy.
The sixth-grade students liked volunteering their time to read to the elderly people in the hospital. It made the volunteers happy.
The sixth-grade students liked volunteering their time to read to the elderly people in the hospital. They made them happy.
The sixth-grade students liked volunteering their time to read to the elderly people in the hospital. Reading made them happy.