Final answer:
Diabetes mellitus is a chronic disorder characterized by inadequate activity of insulin, resulting in high blood glucose levels. The lacking hormone is insulin, and in Type 1 diabetes, the immune system attacks and destroys the insulin-secreting beta cells of the pancreas, leading to a deficiency of insulin.
Step-by-step explanation:
Diabetes mellitus is a chronic disorder characterized by inadequate activity of insulin, resulting in high blood glucose levels. It is not an autoimmune disorder, cardiovascular disease, lung disorder, or kidney condition, as mentioned in the options. The lacking hormone in diabetes mellitus is insulin. In Type 1 diabetes (which is an autoimmune disease), the immune system attacks and destroys the insulin-secreting beta cells of the pancreas, leading to a deficiency of insulin. This necessitates the administration of synthetic insulin by injection or infusion.