The correct option is C ATP increases in muscles.
Rigor mortis is the state of rigidity that develops in the muscles after death. It is used to determine or estimate the time of death. This refers to the stiffening of the body after death due to loss of Adenosine Triphosphate (ATP) from the body's muscles. ATP provides the energy to the muscles which help them to work and without this, the muscles become stiff and inflexible.