Answer:
A. Change student's to students'.
Step-by-step explanation:
In sentence 8, the word student's should be changed to students'. The given text tells about a woman who became a nun and was sent to be a teacher in Calcutta, India. Every teacher has multiple students, and the text doesn't specify that Agnes was concerned about a particular student's education. This leads us to the conclusion that she was concerned about all of her students. The word student's would suggest that she was concerned about one student, while students' suggests that she was concerned about more of them. This is why option A is the correct one.