Review 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 third part, Professor Marshall refused to do more than scowl and read from his yellowed lecture notes. This part has a main clause: Professor Marshall is the subject; refused is the verb. A fragment will not have a main clause.