**Core Concept**
Hemoglobin S (HbS) is a variant of hemoglobin that causes sickle cell disease. The basic defect in HbS is related to its structure and function, specifically how it interacts with its environment.

**Why the Correct Answer is Right**
The correct answer is B. Altered solubility. The basic defect in HbS is due to a point mutation in the beta-globin gene, resulting in the substitution of glutamic acid with valine at position 6 of the beta-globin chain. This mutation reduces the solubility of deoxygenated hemoglobin, causing it to polymerize and form fibers that distort the red blood cell into a sickle shape. This altered solubility is the primary cause of the pathophysiology of sickle cell disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Altered function is incorrect because while HbS does have altered function, this is a consequence of its altered solubility, not the basic defect itself.

**Option C:** Altered stability is incorrect because HbS is actually more stable than normal hemoglobin when deoxygenated, but this stability is a result of its altered solubility, not the primary defect.

**Option D:** Altered O2 binding capacity is incorrect because HbS has a normal oxygen binding capacity, but its altered solubility causes it to polymerize and form fibers when deoxygenated.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the basic defect in HbS is related to its altered solubility, which leads to polymerization and sickling of red blood cells. This understanding is crucial for diagnosing and managing sickle cell disease.

**✓ Correct Answer: B. Altered solubility**