Final answer:
The correct answer is a. regulated variable, sensor. A sensor detects changes in regulated variables like temperature or glucose levels and sends this information to a control center which responds to maintain homeostasis.
Step-by-step explanation:
Changes in the external environment alter the regulated variable, which is detected by the sensor, and that information is sent to the integrator. Therefore, the correct answer to this question is a. regulated variable, sensor. When a change occurs in an animal's environment, receptors (or sensors) detect a stimulus, such as a temperature or glucose level deviation. The sensor then relays this information to a control center, often the brain, which compares the incoming data with the normal values or the set point. If the variable is not within the normal range or has deviated from the set point, the control center coordinates a response to bring the variable back within the normal range, typically involving an effector such as a gland or muscle. This action is part of a homeostatic mechanism and often operates through a negative feedback loop that acts to negate the deviation and maintain homeostasis.