The correct answer is option 3: A is true but R is false. Here`s why:

Assertion (A) is true because one of the main symptoms of diabetes insipidus is extreme thirst. Diabetes insipidus is a condition where the body can`t properly regulate its fluid levels. The patient`s kidney either can`t respond to vasopressin (anti-diuretic hormone) or the pituitary gland isn`t secreting enough of it. This hormone normally tells the kidneys to conserve water, but in diabetes insipidus, its function is disrupted, leading to frequent urination and in turn, increased thirst.

The Reason (R) is false because a person with diabetes insipidus actually suffers from a lack of vasopressin or from a response failure to it, not from excess secretion. Therefore, the thirst in a patient with diabetes insipidus is caused by a lack of or a decreased response to vasopressin, not an excess of it.