Final answer:
The answer is 'd. regulated variable, sensor.' When environmental changes occur, they alter the regulated variable, which is then detected by the sensor, and information is transmitted to the control center to initiate a compensatory response.
Step-by-step explanation:
Changes in the external environment alter the regulated variable, which is detected by the sensor, and the information is sent to the integrator. Therefore, the correct answer is d. regulated variable, sensor. In the context of homeostasis, a change in the environment results in a deviation of a regulated variable from its normal range or set point. When this occurs, sensors pick up this change and relay the information to the control center (often the brain), which then assesses this information and initiates a response to bring the regulated variable back to its set point through effectors, such as muscles or glands, completing the feedback loop.
Negative feedback loops are those in which the response of the system is to reverse the direction of the change, thereby maintaining homeostasis. An example of this is when an endothermic animal, which is able to regulate its own body temperature, experiences a drop in environmental temperature and responds by generating heat to maintain a stable internal temperature.