Although his original theory of personality has been and continues to be criticized, Sigmund Freud’s ideas served to inspire the work of other personality theorists who made significant future contributions to the field. In this sense, Freud’s theory could be considered "useful" because it:

a. stimulated the discovery of new knowledge.
b. suggested that personality is shaped by behaviors that appear to fit together.
c. allowed researchers to predict future events with relative accuracy.
d. provided a comprehensive framework to explain known facts.

Sigmund Freud was the developer of psychoanalysis. Psychoanalysis is the method that helps in the treatment of mental illnesses and the study of human behavior. In this method of mental illness treatment, the person's repressed emotions and thoughts are made conscious thus helping them to come from unconscious to conscious state of mind.

Though his theories of personality development are criticized but it helped in the development of new knowledge of psychoanalysis. His work's helped in the development of further new theories and studies in the same field of knowledge.

So, the correct answer is option A.