Which changing external condition triggers thirst? lack of oxygen lack of water disease-causing agents change in temperature

The correct answer is lack of water.

The external condition which triggers thirst is because there is lack of water. Thirst is termed as the craving fluid which results with a major instinct of animal to drink.

It is a mechanism which is essential in balancing of fluid. When there is low volume of water in the body then the osmolite concentration goes high then the brain signals thirst.

When body is being dehydrated it is when we feel thirst. When there is no proper amount of water in the body or excessive thirst it can results to diabetes mellitus which is high blood pressure.

The correct answer is option D, change in temperature

Reason -

The climatic change affects all our metabolic processes. During winter when the temperature is low, the daily rate of dehydration becomes extremely low in our body and thus, the rate of sweating also reduces due to which the requirement of water in our body gets minimal.  

While in summer, when the external temperature is at its peak condition, the rate of dehydration becomes extremely high and thus the rate of sweating increases which in turn enhances the requirement of water in our body as we get thirsty very fast.  

"do a testcross with a homozygous recessive plant."