**Core Concept**
The underlying principle being tested is the genetic basis of retinoblastoma, a malignant tumor of the retina. Retinoblastoma can occur in heritable or sporadic forms, with the heritable form often associated with a genetic mutation in the **RB1 gene**. Understanding the distinction between these forms is crucial for prognosis and family screening.

**Why the Correct Answer is Right**
To determine if a retinoblastoma is heritable or sporadic, one must identify if there's a mutation in the **RB1 gene**, which is a tumor suppressor gene. The heritable form typically presents with bilateral tumors and has a positive family history, but a unilateral presentation does not rule out heritability.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it is not specified.
**Option B:** Similarly, without the option text, we cannot assess its validity.
**Option C:** And option C is also unspecified.
**Option D:** Option D's correctness cannot be evaluated without the provided text.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that even if a patient presents with unilateral retinoblastoma, there is still a possibility of it being heritable, especially if there's a family history. Genetic testing for the **RB1 gene** mutation can help differentiate between heritable and sporadic forms.

**Correct Answer:** D. Genetic testing for RB1 gene mutation.