At any time during the lifecycle of an assignment, you can go into the assignment settings and update the "Solution Release Settings". Check the boxes to release the solution code and answers to multiple choice, checkbox, code review, and code questions.
When releasing grades, you will be presented with a confirmation modal that will display the settings you specified and give you an opportunity to change your settings before releasing grades.
Note: The action to release solutions is irreversible once grades are released. You cannot remove students' ability to view solutions after grades are released.