You could use a different style for the answer and just change it's attributes to highlight it.

Or you could put the answers into a content control so that each one is defined and then you could use a macro to highlight the content controls or add the checkmark.

These methods are both putting clues in the Word document that smart students could work out but if the test goes out as a PDF or as a printed sheet then they would be fine.

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Andrew Lockton
Chrysalis Design, Melbourne Australia