**Core Concept:**

The correct answer is referring to the normal range of fetal hemoglobin (HbF) percentage in adults, specifically in individuals with sickle cell trait (heterozygous for HbS mutation). Fetal hemoglobin is present during fetal development and normally starts to decrease after birth and is replaced by adult hemoglobin (HbA). However, in individuals with heterozygous sickle cell trait, the percentage of fetal hemoglobin remains relatively high due to the presence of the sickle cell trait.

**Why the Correct Answer is Right:**

Hemoglobin F (HbF) is the fetal form of hemoglobin that is present in a normal adult human. In individuals with sickle cell trait, which is caused by the presence of both normal hemoglobin (HbA) and sickle cell hemoglobin (HbS), the percentage of fetal hemoglobin remains relatively high. This is because the production of HbA is suppressed in individuals with HbS, and the normal fetal hemoglobin production is not suppressed. As a result, the HbF percentage remains elevated compared to normal adults without HbS.

**Why Each Wrong Option is Incorrect:**

A) The percentage of fetal hemoglobin in a normal adult (without HbS) is typically around 5-10%, which is much lower than the percentage in individuals with sickle cell trait.

B) The percentage of fetal hemoglobin in a sickle cell anemia patient would be even higher than in individuals with sickle cell trait, as the production of HbA is completely suppressed in sickle cell anemia.

C) Fetal hemoglobin is not typically present in a person with thalassemia, as thalassemia is a genetic disorder affecting the production of globin chains and not specifically the presence of fetal hemoglobin.

D) The percentage of fetal hemoglobin in an individual with beta-thalassemia trait, a genetic disorder affecting globin chain synthesis, is not significantly higher compared to normal adults without the trait.

**Clinical Pearl:**

In the context of genetic hemoglobinopathies, understanding the percentage of fetal hemoglobin is important for interpreting laboratory results and making accurate diagnoses. The presence of high fetal hemoglobin levels can indicate a specific genetic disorder, such as sickle cell trait or beta-thalassemia, helping clinicians to make an accurate diagnosis. In contrast, a normal adult typically has a lower percentage of fetal hemoglobin.