**Core Concept**
Tuberous sclerosis complex (TSC) is a genetic disorder caused by mutations in either the TSC1 or TSC2 gene, leading to the development of hamartomas and other tumors in various organs.

**Why the Correct Answer is Right**
The correct answer is an example of germline mosaicism, where one parent has a mixture of normal and mutated cells in their reproductive cells (germline). This means that some of the parent's sperm or eggs are normal, while others carry the TSC1 or TSC2 mutation. In this case, both parents appear normal because they are phenotypically normal, but they can still pass the mutated gene to their offspring. The two affected children likely inherited the mutated gene from an affected parent who is a mosaic.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it implies that the parents are carriers of a recessive allele, but the parents are phenotypically normal, indicating that they do not carry a recessive allele for TSC.
* **Option B:** This option is incorrect because it suggests that the parents have a de novo mutation, but the fact that both children are affected suggests that the mutation is inherited from one of the parents.
* **Option D:** This option is incorrect because it implies that the parents are affected with TSC, but the question states that both parents are normal.

**Clinical Pearl / High-Yield Fact**
Germline mosaicism can occur in various genetic disorders, leading to unexpected inheritance patterns. It is essential to consider this possibility when evaluating families with affected children and normal parents.

**Correct Answer: C. Germline mosaicism.**