What is the role of the effector in a feedback loop?
a. it detects a change due to a stimulus.
b. it causes a response to a stimulus
c. it is the respone to a stimulus

The receptor monitors the set point for a variable in the environment. If there is a a stimulus, the receptor signals the control center to produce an effector that will alter the environment and bring it to the set point.
Hence,the right answer is :
c. it is the respone to a stimulus