When Jenny saw that a large barking dog was enclosed by a very high fence, her racing heartbeat began to slow. The slowing of her heartbeat was most directly regulated by her

A) hypothalamus.

B) parasympathetic nervous system.

C) somatic nervous system.

D) sympathetic nervous system.

Answer: B) Parasympathetic nervous system

Explanation:

Parasympathetic nervous system is the part of nervous system that performs the function of conserving body energy and improving gland and digesting action of body. It slows down the heart rate and intensity of body so that body can go into relaxing mode.

According to the question, Jenny's heartbeat has slowed down after seeing the dangerous dog is inside the fenced wall. This phenomena shows that parasympathetic nervous system is processing the process.

Other options are incorrect because somatic nervous system induces voluntary movement of body parts. Sympathetic nervous system increases intensity of body for task and hypothalamus is used for brain activity for function like releasing hormones in body etc.

Thus the correct option is option (B).