An ice fisherman has fallen through the ice. in response to the cold water, sensors in the man's skin send a "cold" signal to the in the man's brain, which in turn sends signals to muscle cells to begin to contract. as a result of this process, the fisherman begins to shiver.

Hypothalamus

Explanation:

The hypothalamus is a part of the brain which possesses temperature receptor cells that detect changes in the man’s temperature, thereby sending signals in the form of electrical nerve impulses to the man’s muscles and nervous system, which in turn respond in counteracting the drop in the normal temperature of the body.  

Once the muscle cells of this man receive these signals, they produce heat through thermogenesis by shivering when the muscle cells begin to contract. This is one of the mechanisms by which thermoregulation is achieved as controlled by the hypothalamus in the brain of the man.

correct answer is a. and d.