Which changing external condition triggers the thirst?

Lack of water triggers thirst.

The lack of water in the external environment creates an urge to drink water. The mind sends the signal that there is a loss of water from the body and it is dehydrated. This signals is recognized by the body and water is taken after this signal. The human body is made up of 70% of water and is vital for performing various functions of the body. Dehydration of water can led to several problems. The human body should not wait for the signal associated to dehydration and 4-5 liters of water must be taken daily.

is this a psat question, if so do not cheat try your best.