THIRST - 'Thirst' is a noun, which means 'need for water/drink'. For example 'I get a terrible thirst after running.'
Option B - 'Thirstily' is the adverb form of the noun 'Thirst', which means 'in a thirsty manner.' For example, 'I drank the water thirstily.' Hence, option B is incorrect.
Option C - 'Hungry' is the adjective form of the noun 'hunger', which means 'to be needing food.' Hence, option C is incorrect.
Option D - 'Dehydrator' is a noun, which means 'a person or a thing that dehydrates.' As it is not the adjective form of the noun 'thirst', it is a wrong choice. Hence, option D is incorrect.
Option A - 'Thirsty' is the adjective form of the noun 'thirst'. For example, 'I a very thirsty.' Hence, option A is the correct answer.