Once the body has begun shivering, what happens to make it stop shivering? a) when body temperature returns to normal, the respiratory system sends a message to the skeletal system to stop shivering. b) when body temperature returns to normal, the nervous system sends a message to the muscular system to stop shivering. c) when body temperature is below normal, the nervous system sends a message to the skeletal system to stop shivering. d) when body temperature is below normal, the respiratory system sends a message to the muscular system to stop shivering.

The answer is B).
One hint is that, if a message is being sent, the "sender" is probably the nervous systems. There are several subsystems to the nervous system, but it regulates and controls virtually all bodily functions. The nervous system monitors your body's temperature, and takes actions to regulate it, such as telling your muscular system to shiver to generate more heat. It sends this message to the muscular system because your muscles cause bodily movement, such as walking, lifting weights, and even shivering. 

When your nervous system determines that your body temperature has returned to normal, it again sends a message to your muscular system to stop shivering. 
Answer is B

B is the correct answer

4, two dominant and two recessive