Explanation

The function to give a message to effectors is b. Negative feedback. 

Negative feedback: In a negative feedback loop, a sensor detects a change in a controlled variable (like temperature) and sends a signal to a control center (like the hypothalamus).

The control center then sends a signal to an effector (like a sweat gland or muscle) to bring the variable back to its set point. Essentially, the effector's response is opposite to the initial change.