**Core Concept**
Rhesus incompatibility is a condition that occurs when a Rh-negative mother is pregnant with a Rh-positive fetus, leading to potential hemolysis of the fetus's red blood cells. The management of Rh-negative mothers is crucial to prevent alloimmunization and ensure the well-being of the fetus.

**Why the Correct Answer is Right**
The next step in the management of a Rh-negative mother is to administer Rh immunoglobulin (RhIg) to prevent the formation of antibodies against Rh-positive red blood cells. RhIg works by coating the Rh-positive red blood cells and preventing them from stimulating an immune response. This is a crucial step in preventing alloimmunization and ensuring the well-being of the fetus. The administration of RhIg is typically done at 28 weeks of gestation and within 72 hours of delivery if the mother has a Rh-positive fetus.

**Why Each Wrong Option is Incorrect**
**Option A:** Administration of RhIg is not necessary if the mother is already immune to Rh-positive red blood cells. This is not the case for a G1+O (gravida 1, para 0) patient attending the antenatal clinic for the booking visit.
**Option B:** Fetal blood sampling is not a necessary step in the management of a Rh-negative mother.
**Option C:** Ultrasound examination is not a direct step in the management of Rh-negative mothers, although it may be used to determine fetal well-being.

**Clinical Pearl / High-Yield Fact**
Rh immunoglobulin should be administered to all Rh-negative mothers at 28 weeks of gestation and within 72 hours of delivery if the mother has a Rh-positive fetus. This is a crucial step in preventing alloimmunization and ensuring the well-being of the fetus.

**Correct Answer: D. Administration of Rh immunoglobulin (RhIg) at 28 weeks of gestation and within 72 hours of delivery if the mother has a Rh-positive fetus.**