The most suitable option that would provide the basis for a legal claim against the directors of Enron is:
e. The directors did not exercise their best business judgment.
This option implies a failure of the directors to fulfill their fiduciary duty to act in the best interests of the shareholders. Directors have a responsibility to make informed and reasonable decisions in their oversight of the company's management and operations. If they failed to do so, it could be argued that they are liable for any resulting damages to the shareholders.
While the other options may suggest areas of concern regarding the behavior of the directors, they do not directly indicate a breach of fiduciary duties as strongly as option (e).