Which term describes a mechanism by which the internal conditions of an organism are kept at set values without regard to the external conditions?

The answer is homeostasis. 

Homeostasis is a mechanism which controls internal conditions of an organism so that internal conditions remain stable without regard to the external conditions. So, it regulates body temperature, water level, sugar level, etc. It works through negative feedback mechanism. For example, if body temperature is increased, the body detect the change and activate mechanisms that decrease body temperature. Without homeostasis, the organisms could not survive.

Ichoose a as my answer