The correct answer to the question is option 4, Vasopressin. Diabetes insipidus is a condition characterized by excessive thirst and production of large amounts of diluted urine. It occurs due to a deficiency or lack of response to the hormone vasopressin, also known as antidiuretic hormone (ADH).

Vasopressin is produced in the hypothalamus and stored in the posterior pituitary gland. It plays a crucial role in regulating the balance of water in the body. When released, vasopressin acts on the kidneys, where it increases water reabsorption and reduces urine output.

If there is a deficiency or absence of vasopressin, the body loses excessive amounts of water through urine, leading to the symptoms of diabetes insipidus. This can be caused by various factors, including damage to the hypothalamus or pituitary gland, certain medications, or genetic disorders.

It is important to note that diabetes insipidus is different from diabetes mellitus, which is characterized by high blood sugar levels due to a deficiency or resistance to the hormone insulin. Therefore, option 3, Insulin, is not the correct answer for this question.