The most common cause of Addison's disease is -: (A) Autoimmune adrenalitis, (B) Meningococcal septicemia, (C) Malignancy, (D) Tuberculosis
Answer is A. Most common cause (60%-70%) of primary adrenocorticol insufficiency (Addison'discases) is autoimmune adrenelitis.
Other common cause are.

Tuberculosis → Most common cause of Addison's disease in India.
AIDS
Metastatic cancers