Final answer:
The stiffening of the body that begins at the neck and lower jaw is called rigor mortis, which is a condition occurring after death due to the myosin heads not detaching from actin-binding sites. Option 1 is correct.
Step-by-step explanation:
The term used to describe the stiffening of the body that starts at the neck and lower jaw and spreads downward is rigor mortis. Rigor mortis is a condition that occurs after death, where the myosin heads will not detach from the actin-binding sites, resulting in muscle stiffness.
Although conditions such as tetanus and Parkinson's disease can also cause muscle rigidity, the specific pattern of stiffening in rigor mortis clearly distinguishes it from the sustained muscle contractions seen in tetanus or the hypertonia characteristic of Parkinson's disease.