While grading a test, you can see how frequently students gave which answer. This feature works only for certain types of exercises. (Currently it is enabled for Open, Text, Dropdown, Essay, Radio Button (random) and Multiple Choice (random).
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Go to the Test results, and then to the tab "Result per question". In the column "Answer frequency" you will see a button if the frequency feature is available.

When you click on the button, a window will open up and show the frequency table.

Note that the exercise texts that are shown do not contain any randomised values, since these would be different per attempt.