Assertion-Reason relation
Diabetes insipidus is a condition related to water imbalance in the body. In this condition, the kidney can’t concentrate urine properly.
Due to this the person urinates more often than usual. It creates a demand for more water and the person experiences excessive thirst (polydipsia).
Hence, the assertion is true.
The deficiency of ADH or vasopressin in the body also causes excessive loss of water through urine (diuresis). This also gives rise to diabetes insipidus. So, the reason statement is incorrect.