Which is not an autoimmune disease?

The correct option is B Insomnia
Autoimmunity :
The condition of the body in which the body fails to differentiate between self and non-self cells and start attacking self cells is called autoimmunity.
Examples include rheumatoid arthritis, pernicious anaemia, Grave’s disease etc.

Grave’s disease :
Grave’s disease is an autoimmune disease that results in hyperthyroidism (overproduction of thyroid hormones). Common symptoms include anxiety, irritability, heat sensitivity, weight loss etc.

Pernicious anaemia :
Pernicious anaemia is an autoimmune disease characterised by the inability of the body to properly utilise vitamin B$12$, which is essential for the development of red blood cells. Common symptoms include fatigue, shortness of breath, jaundice, loss of appetite, diarrhoea, bleeding gums etc.

Rheumatoid arthritis :
Rheumatoid arthritis is an autoimmune disease in which the immune system attacks the joints. Common symptoms include joint pain and swelling.

Insomnia :
Insomnia is a sleep disorder which makes it hard to fall asleep. It is not an autoimmune disease.

Final answer :

(d) Insomnia