Which field can be the STUDENT table$$s primary key?

The following image shows a snapshot of four tables in a database.

STUDENT table: The attributes are STU$\_$ID$,$ STU$\_$LNAME, STU$\_$FNAME, and STU$\_$EMAIL

ENROLLMENT table: The attributes are STU$\_$ID$,$ COURSE$\_$CODE, and STU$\_$GRADE

COURSE table: The attributes are COURSE$\_$CODE, COURSE$\_$NAME, and FAC$\_$ID

FACULTY table: The attributes are FAC$\_$ID$,$ FAC$\_$LNAME, FAC$\_$FNAME, FAC$\_$RANK