Rigor mortis is `:-`

Options

A. Contraction of muscles after death
B. Contraction of muscles before death
C. Shivering of muscles
D. None

Answer

### Step-by-Step Solution:

1. **Understanding Muscle Contraction and Relaxation**:
- Muscle contraction involves the interaction between actin and myosin filaments, which requires adenosine triphosphate (ATP) to form the actomyosin complex.

2. **Role of ATP in Muscle Function**:
- ATP is essential not only for muscle contraction but also for muscle relaxation. During relaxation, ATP is needed to actively pump calcium ions back into the sarcoplasmic reticulum, which is crucial for muscle fibers to relax.

3. **Effect of Death on Muscle Activity**:
- After death, ATP production ceases. This lack of ATP means that calcium ions cannot be pumped back into the sarcoplasmic reticulum, leading to a sustained contraction of the muscles.

4. **Definition of Rigor Mortis**:
- The stiffening of the muscles after death due to the inability to relax is referred to as rigor mortis.

5. **Duration of Rigor Mortis**:
- Rigor mortis typically begins a few hours after death and will eventually soften as the muscle tissue starts to degenerate.

6. **Conclusion**:
- Therefore, the correct answer to the question "Rigor mortis is" is the stiffening of the muscles after death.