By default, learners who have submitted incorrect responses to assessment questions are able to elect to see the correct answer.
This can be switched off, preventing learners from viewing the correct response, though they will still know if they have answered correctly or incorrectly (unless you want to switch that off too).
To prevent learners from viewing the correct answers:
1. Navigate to the storyboard view of your learning object.
2. Select the 'Settings' in the top right-hand of your screen.
3. Select the 'Assessment' tab.
4. Under the 'Feedback' heading, toggle the option 'Show correct answers after each question' to 'No'.