Jack bends and straightens his arm during arm wrestling. Which type of muscle does Jack use in the process?

Skeletal muscle

Explanation:

Skeletal muscle refers to the muscles that are attached to our bones.  These muscles were attached by a collection of collagen fibers which most commonly known tendons. We can move the vast majority of these muscles voluntarily

The act of bending and straightening the elbow like Jack did is being done by using the combination of biceps and triceps muscle that are attached to the skeletal structure of your arm.

would not cooperate with each other voluntarily.