The correct answer is d) receptor.
In homeostatic control mechanisms, receptors play a crucial role as they are the components that monitor the environment. They detect changes or stimuli in both internal and external conditions. Once a stimulus is recognized by the receptor, this information is sent to the control center, which then processes the information and decides on the appropriate response. The effector is the component that carries out the response to restore balance, but it is the receptor that initially senses the change. Therefore, in the context of monitoring environmental changes, the receptor is essential.