**Core Concept**
The inquest into a custodial death is a legal inquiry conducted to determine the cause and circumstances surrounding the death of an individual in police custody. This process is governed by the law and is designed to ensure accountability and transparency in the handling of such cases.

**Why the Correct Answer is Right**
In the case of a custodial death, the law requires that an inquest be conducted by a Magistrate. This is because a Magistrate has the authority to investigate and gather evidence, and to make recommendations for further action. The Magistrate's role is to ensure that the investigation is thorough and impartial, and that the rights of the deceased and their family are protected. The Magistrate may also summon witnesses, including police officials and medical experts, to testify in the inquest.

**Why Each Wrong Option is Incorrect**
**Option A:** A Doctor is not qualified to conduct an inquest, as they are not empowered to investigate crimes or gather evidence.
**Option C:** A Sub-inspector is a police officer who may be involved in the case, but they are not authorized to conduct an inquest.
**Option D:** A District attorney may be involved in prosecuting cases related to custodial deaths, but they are not responsible for conducting the inquest.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the inquest into a custodial death is a critical step in ensuring that the rights of the deceased and their family are protected, and that those responsible for their death are held accountable.

**✓ Correct Answer: B. Magistrate. A Magistrate is responsible for conducting an inquest into a custodial death.**