Final answer:
The component that monitors the environment in a homeostatic control mechanism is the receptor, which senses changes and relays information to the control center. The control center then activates an effector if necessary to maintain homeostasis through negative feedback.
Step-by-step explanation:
In a homeostatic control mechanism, the component that monitors the environment is known as the receptor. The receptor is responsible for detecting any changes in the environment or within the body. Once a change is detected, the receptor sends information to the control center, usually the brain in most animals or the spinal cord, which then evaluates the change and determines the necessary response. If the change is significant and needs adjusting, the control center sends signals to the appropriate effector, which is typically a muscle or gland. The effector then takes action to adjust the variable back towards the set point, thus maintaining homeostasis through a process known as negative feedback.