**Core Concept**
A magistrate inquest is a legal inquiry conducted by a magistrate to determine the cause of an unnatural or suspicious death. This process is essential in India, as per the Code of Criminal Procedure, 1973, to investigate and establish the facts surrounding the death.

**Why the Correct Answer is Right**
The correct answer is related to the circumstances under which a magistrate inquest is mandatory. When a person dies under suspicious or unnatural circumstances, such as in a police custody, prison, or while in the custody of another public servant, a magistrate inquest is required to be conducted. This ensures that the investigation is conducted impartially and that the facts surrounding the death are established accurately.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a magistrate inquest is not mandatory in cases of death due to natural causes.
**Option B:** This option is incorrect because a police inquest is generally conducted in cases of deaths that occur in a hospital or any other place where medical aid is available, whereas a magistrate inquest is for suspicious or unnatural deaths.
**Option C:** This option is incorrect because a magistrate inquest is not required in cases of deaths due to riots or communal violence.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that a magistrate inquest is a legal inquiry and not a medical investigation. The primary goal is to determine the cause of death and establish the facts surrounding the event, rather than to determine the medical cause of death.

**Correct Answer: A. Death in police custody.**