If you’re using the questions in an official “test”, it gets quite a bit more involved. For a test, the feedback runs an action group that:
Checks the answer (question variable value) and IF it contains the “secondary” correct answer, modifies a custom variable to hold that choice
Checks the answer (question variable value) and IF it contains the “secondary” correct answer, changes it to the answer selected in question properties
Processes the question
Displays a message indicating if the answer was correct or not
If you need a results page for the test, you will need to use a custom results page. Why? Because the built-in results page processes the test and reads the question answers before you have a chance to change them back to what the student ACTUALLY selected. If you change it to what the student answered as part of the group on each question page, the results will be incorrect.
With the custom results page, you:
Process the test
Switch the question answers to the one the student actually selected based on the custom variable
Use a text box or whatever you wish to display as part of the results.
I attached a sample that uses both immediate processing and using a submit button for submitting the answer. There’s also a sample custom results page at the end.
Another way to achieve it would be to rebuild the single choice question using a radio button group. Make each radio button modify the variable of an invisible “Fill in the Blank”-question to equal the groups variable. Enter all correct options as correct answers. The radio buttons are not considered for the test but the question can normally be used with all feature in a test or not.