The student has decided on an answer to this question and has marked the question as completed via the "Mark as Completed" checkbox at the bottom of the question panel. Thus, the question number panel border has changed from yellow to green, indicating completion.
This merely serves as a visual reminder to the student, and in no way "locks in" the student's answer. The student may, if he or she wishes, neglect to mark any answers as completed, and still submit the test as normal upon completion.