**Core Concept**
A Robesonian translocation involves a balanced exchange of genetic material between two acrocentric chromosomes, typically chromosome 21 and another acrocentric chromosome, such as chromosome 14, 22, or 15. This results in a carrier state for the translocation, who may be phenotypically normal but has an increased risk of producing chromosomally abnormal gametes.

**Why the Correct Answer is Right**
In a Robesonian translocation carrier, each gamete has a 50% chance of inheriting the translocated segment. When a translocation carrier's ovum is fertilized by a normal sperm, there is a 50% chance that the resulting zygote will inherit the translocated segment from the ovum and a 50% chance that it will inherit a normal chromosome 21. If the translocated segment is inherited, it can result in a functional trisomy 21 due to the duplication of genes on chromosome 21. Therefore, the theoretic likelihood of a functional trisomy 21 is 50%.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately reflect the likelihood of a functional trisomy 21 in the offspring of a Robesonian translocation carrier.

**Option B:** This option is incorrect because it overestimates the likelihood of a functional trisomy 21.

**Option C:** This option is incorrect because it underestimates the likelihood of a functional trisomy 21.

**Option D:** This option is incorrect because it does not accurately reflect the likelihood of a functional trisomy 21 in the offspring of a Robesonian translocation carrier.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that Robesonian translocation carriers have a 50% risk of passing on the translocated chromosome to their offspring, resulting in a functional trisomy 21. This highlights the importance of genetic counseling and prenatal testing in high-risk pregnancies.

**Correct Answer: D. 50%.**