**Core Concept**
Immediate stiffness postmortem refers to the rapid onset of rigidity after death, which is a crucial aspect of forensic medicine. This phenomenon is often observed in specific circumstances and is essential for forensic investigators to differentiate from other postmortem changes.

**Why the Correct Answer is Right**
Cadaveric spasm, also known as instantaneous rigor, is a rare but well-documented phenomenon characterized by the instantaneous contraction of muscles at the moment of death. This results in the body retaining the position it was in at the time of death, often with the hands in a grasping or clenched position. The exact mechanism of cadaveric spasm is not fully understood, but it is thought to be related to the sudden disruption of neural activity and the resulting contraction of muscles. Cadaveric spasm is an important diagnostic feature in forensic pathology, as it can provide valuable information about the circumstances surrounding death.

**Why Each Wrong Option is Incorrect**
**Option A:** Cadaveric rigidity is a term sometimes used interchangeably with cadaveric spasm, but it is not a specific medical term and does not accurately describe the phenomenon in question.

**Option C:** Rigor mortis is a well-known postmortem change characterized by the stiffening of muscles due to the breakdown of ATP and the resulting contraction of muscles. However, it typically develops several hours after death, not immediately.

**Option D:** Algor mortis refers to the cooling of the body after death, which is a gradual process that occurs over several hours. It is not related to the immediate stiffness postmortem.

**Clinical Pearl / High-Yield Fact**
It's essential for forensic medicine practitioners to be aware of the distinction between cadaveric spasm and rigor mortis, as the former can provide crucial information about the circumstances surrounding death, while the latter is a more general postmortem change.

**✓ Correct Answer: B. Cadaveric spasm**