**Core Concept**
The ABO blood group system is determined by the presence or absence of A and B antigens on the surface of red blood cells. The Rh blood group system is determined by the presence or absence of the RhD antigen. A person's blood group is determined by the combination of these antigens. If a person has the O blood group, it means they are lacking both A and B antigens. If they are Rh negative, it means they lack the RhD antigen.

**Why the Correct Answer is Right**
A baby inherits one set of ABO and Rh blood group genes from each parent. Since the baby is Rh negative, they must have inherited two Rh negative genes (one from each parent). This means that both parents must contribute one Rh negative gene to the baby. However, the Rh negative gene is recessive, meaning that a person can be Rh positive if they have one Rh positive and one Rh negative gene. If a parent is Rh positive, they can only contribute an Rh positive gene to the baby. Therefore, the parent who is Rh negative must have inherited two Rh negative genes from their own parents, and this parent must be homozygous recessive for the Rh blood group gene.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a person can be Rh positive if they have one Rh positive and one Rh negative gene.

**Option B:** This option is incorrect because a person can be Rh positive if they have one Rh positive and one Rh negative gene.

**Option C:** This option is incorrect because a person can be Rh positive if they have one Rh positive and one Rh negative gene.

**Option D:** This option is incorrect because a person can be Rh positive if they have one Rh positive and one Rh negative gene.

**Clinical Pearl / High-Yield Fact**
In the ABO blood group system, the O blood group is considered universal donor because their red blood cells lack both A and B antigens, making them compatible with all other blood groups.

**Correct Answer:** B.