Answer:
The alternatives to this question are:
A. It will act as a complete defense to an action for defamation.
B. It will establish that the man has not suffered any actual injury.
C. It may diminish the damages that the man would be entitled to recover.
D. It proves that the girlfriend had no reasonable ground for believing that the man was fired for dishonesty.
The correct answer is the letter C.
In the case that the man sues his former girlfriend this would not really indicate a complete defense but would help him diminish the damages that he would entitle to recover.