**Core Concept**
Histopathology is the most critical determinant of prognosis in Wilms tumor, as it directly reflects tumor biology, grade, and aggressiveness. The classification of Wilms tumor into favorable, intermediate, and unfavorable subtypes based on histological features (e.g., blastemal, papillary, or clear cell components) strongly predicts outcome and guides treatment intensity.

**Why the Correct Answer is Right**
Histopathology provides the most reliable indicator of tumor behavior and survival. It allows differentiation between favorable (e.g., favorable histology with well-differentiated cells) and unfavorable subtypes (e.g., anaplastic or rhabdomyoblastoma-like components), which are strongly associated with recurrence and mortality. This classification is used in international guidelines (e.g., International Society of Pediatric Oncology) to stratify patients for therapy. Unlike ploidy or genetic mutations, histopathology is routinely available, reproducible, and directly linked to clinical outcomes.

**Why Each Wrong Option is Incorrect**
Option B: Ploidy of cells is a secondary factor and not routinely used in clinical decision-making. While aneuploidy is associated with poor prognosis, it is less reliable than histopathology due to variability and technical challenges.
Option C: Age < 1 year is a risk factor for tumor development but not a strong predictor of prognosis. Infants have higher risk of relapse, but histopathology remains the most accurate prognostic tool.
Option D: Mutation of c1p gene (likely a typo; possibly referring to *WT1* or *TP53*) is not a primary prognostic factor. While *WT1* mutations are linked to tumor development, they are not as clinically actionable or widely used as histopathology.

**Clinical Pearl / High-Yield Fact**
In Wilms tumor, "favorable histology" is the strongest independent predictor of survival—patients with this subtype have significantly better outcomes and may avoid aggressive therapy. Always assess histopathology in every case.

✓ Correct Answer: A. Histopathology