For a multiple-choice exam, to ease the grading procedure, it is often preferred to ask students to write their answers collectively in an answer sheet with their choices of answers filled in boxes. However, LaTeX, in particular the exam document class, does not directly provide the feature to automatically generate such boxes. In this post, we will let LaTeX to automatically generate these answering boxes, and with correct answers filled in when the answers document class option is turned on. The effects are displayed below, with correct answers shown and not shown, respectively. Their respective PDF files are also available: Without answers; with answers.
Below is the minimum working example corresponding to the effects shown above, with key lines highlighted. (The source file can also be downloaded here.)