Your heart works by: - almost entirely involuntary muscle contraction, but sometimes voluntary contraction.

-entirely voluntary muscle contraction.

- a balanced mix of voluntary and involuntary muscle contraction.

- entirely involuntary muscle contraction.

The correct answer is - involuntary contraction of the skeletal muscles.

Explanation:

Shivering or the chills that is involuntary action of body to maintain the homeostasis by controlling the body heat or maintaining body temperature it can cause by environmental change or the infection.

It is rapid involuntary contraction and relaxation of the skeletal muscles whenever body needs to produce heat when ti feels cold. Skeletal muscles are act as thermal effector in such cases.

Thus, the correct answer is - involuntary contraction of the skeletal muscles.