**Core Concept**
Rigor mortis is a postmortem stiffening of muscles due to the depletion of ATP and subsequent phosphorylation of actin and myosin filaments, leading to fixed muscle contraction. This process begins after death and is a key indicator in forensic medicine for estimating time of death.

**Why the Correct Answer is Right**
Rigor mortis typically starts within 1–2 hours after death, as ATP levels drop and the muscle proteins become locked in a contracted state. This phase is due to the loss of calcium regulation in muscle cells, which causes actin and myosin to bind irreversibly. The onset is most consistent in warm environments and varies slightly with body temperature and muscle type.

**Why Each Wrong Option is Incorrect**
Option A: 1/2–1 hrs – This is too early; rigor mortis usually begins after 1–2 hours, not sooner.
Option C: 3–6 hrs – This is the peak phase of rigor mortis, not its onset.
Option D: 12 hrs – This is when rigor mortis begins to reverse (flaccid phase), not when it first develops.

**Clinical Pearl / High-Yield Fact**
Rigor mortis peaks at 3–4 hours postmortem and begins to resolve after 12–24 hours. Always consider environmental temperature and body condition when interpreting timing—warm climates accelerate rigor mortis onset.

✓ Correct Answer: B. 1-2 hrs