**Core Concept**
Sickle cell anemia is an autosomal recessive genetic disorder caused by a mutation in the HBB gene, leading to the production of abnormal hemoglobin (HbS). This condition is characterized by the presence of two copies of the mutant allele, one inherited from each parent.

**Why the Correct Answer is Right**
When both parents have sickle cell anemia, they are homozygous for the recessive allele (HbS/HbS). Each child has a 25% chance of inheriting two copies of the mutant allele (HbS/HbS), a 50% chance of inheriting one copy of the mutant allele and one normal allele (HbS/Hb), and a 25% chance of inheriting two normal alleles (Hb/Hb). Since the disease manifests only when both alleles are mutant, the likelihood of children having the disease is 25%.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, so we cannot evaluate its correctness.
* **Option B:** This option is not provided, so we cannot evaluate its correctness.
* **Option C:** This option is not provided, so we cannot evaluate its correctness.
* **Option D:** This option is not provided, so we cannot evaluate its correctness.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that autosomal recessive disorders, like sickle cell anemia, have a 25% chance of expressing in offspring when both parents are carriers, and a 50% chance of expressing when both parents are carriers and one of them has the disease.

**Correct Answer:** C. 25%