The sentence says Ann was ill. Thus, it can be concluded that Ann was in a place where ill people are treated and that is the place where everyone went to see her.
Option A is incorrect. School is a place where people go to study. As ill people are not treated at a school, it is incorrect.
Option B is incorrect. Church is a place of worship and not a place where ill people are treated.
Option C is incorrect. University is a place where people go for higher education. Thus, it does not fit in the blank.
Option D is correct. A hospital is a place where doctors treat ill people. It can be said that Ann was in a hospital when she was ill. Thus, it is the place where everyone went to see her.