Nny is running for more than an hour on a pleasant sunny morning. Her body is sweating to maintain homeostasis, which is caused by the action of a/an .

hypothalamus

Explanation:

"Homeostasis" refers to the ability of the organism to adapt to the changing environment in order to be stable.

In the situation above, Jenny has been running under the sun. The "hypothalamus" then senses this high temperature in the body; thus, it sends signals towards the skin's sweat glands in order to lower the temperature. The sweat glands (particularly the eccrine glands) then removes heat by secreting sweat unto the surface of the skin. The sweat then evaporates. This is an example of homeostasis.