Final answer:
Rigor mortis is a possible explanation for an un-flexed, straight arm discovered in a fire. It occurs when the myosin heads in the muscles cannot detach from the actin-binding sites, resulting in muscle stiffness.
Step-by-step explanation:
The correct answer is d) Rigor mortis.
Rigor mortis is a condition that occurs after death when the muscles become stiff and un-flexed. It is caused by the myosin heads in the muscles being unable to detach from the actin-binding sites, resulting in muscle stiffness. While rigor mortis is a natural process that occurs in a deceased person, it can also be observed in a fire if a victim's arm is found un-flexed and straight.
Example: When a person dies, the ATP molecules that are required for muscle contraction are no longer produced, causing the myosin heads to remain attached to the actin-binding sites, leading to muscle stiffness and rigidity.