**Core Concept**
The International Lymphoma Study Group proposed a classification system for non-Hodgkin's lymphoma (NHL) to standardize the diagnosis and treatment of this group of malignancies. This classification system is based on the lymphoma's cell of origin, histological features, and molecular characteristics.

**Why the Correct Answer is Right**
The correct answer is the World Health Organization (WHO) classification, which is widely accepted and used by pathologists and oncologists worldwide. The WHO classification system categorizes NHL into several subtypes, including diffuse large B-cell lymphoma, follicular lymphoma, and mantle cell lymphoma, among others. This classification system is essential for guiding treatment decisions and predicting patient outcomes.

**Why Each Wrong Option is Incorrect**
**Option A:** The Rappaport classification system is an older classification system that was widely used in the past but has been largely replaced by the WHO classification.

**Option B:** The Kiel classification system is another older classification system that was used in Europe but has not been widely adopted globally.

**Option C:** The REAL classification system is an acronym for "Revised European-American classification of lymphoid neoplasms," which is an older classification system that was replaced by the WHO classification.

**Clinical Pearl / High-Yield Fact**
The WHO classification system is updated periodically to reflect advances in our understanding of lymphoma biology and to ensure that the classification system remains relevant and useful for clinical practice.

**Correct Answer:** C. REAL classification system