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
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.