Take another look at the passage:

Making his anxious students relax would have required very little effort. Like a smile when he walked into class or a "Good job" written at the bottom of an essay. Professor Marshall refused to do more than scowl and read from his yellowed lecture notes.

You chose the first part, Making his anxious students relax would have required very little effort. This part has a main clause: Making, a gerund, is the subject; would have required is the verb. A fragment will not have a main clause.

Go back to the passage to try again.

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