In "Examination Day" by Henry Slesar, the knowledge of the exam affects Dickie's mom and dad by causing them stress and concern about their son’s readiness for the exam. Therefore, the correct answer is:
C. Dickie's mom and dad stress about preparing their son for the exam.
Throughout the story, both parents exhibit anxiety regarding the examination that is mandatory for all children turning twelve, highlighting their worries about the implications of the test results for Dickie.