**Core Concept**
The Bombay blood group is a rare blood type characterized by the absence of H antigen on red blood cells, which is responsible for the ABO blood group system. This absence leads to a unique phenotype where individuals with the Bombay blood group are unable to express either A or B antigens on their red blood cells.

**Why the Correct Answer is Right**
The Bombay blood group is due to a homozygous state for the H gene mutation, known as the "Bombay phenotype". This mutation results in a complete absence of the H antigen, which is necessary for the synthesis of A and B antigens. As a result, individuals with the Bombay blood group have a non-reactive phenotype in the ABO blood group system. The H antigen is encoded by the FUT1 gene, which is responsible for the synthesis of the H antigen.

**Why Each Wrong Option is Incorrect**
* **Option A:** Hb refers to hemoglobin, which is not related to the ABO blood group system or the Bombay blood group phenotype.
* **Option B:** Ac is not a recognized antigen in the blood group system.
* **Option D:** D refers to the RhD antigen, which is a separate antigen system and not related to the ABO blood group system or the Bombay blood group phenotype.

**Clinical Pearl / High-Yield Fact**
The Bombay blood group is a rare but important consideration in blood transfusion medicine, as individuals with this phenotype may require special blood typing and cross-matching procedures to avoid adverse reactions.

**Correct Answer: B. Ac**