It depends on where the text is entered.

Teacher-entered text in the student response area of an Open Answer cannot be center-aligned. Any directions or starter text that appear in the response box will remain left-aligned.

Students can center their own text when responding if the Rich Text Editor is enabled (this is on by default for Open Answer questions). Students can use the alignment tools in the rich text toolbar while typing their response.

Teacher Notes can be center-aligned. These notes appear in the scoring view for the teacher only and are not visible to students.