## Core Concept
Restriction Fragment Length Polymorphism (RFLP) is a technique used in molecular biology to analyze and compare DNA samples. It involves the use of **restriction endonucleases** (enzymes that cut DNA at specific sequences) to generate fragments of varying lengths depending on the presence or absence of specific restriction sites. This variation in fragment lengths is used to identify genetic differences between individuals or species.

## Why the Correct Answer is Right
The correct answer involves understanding the basic principles and applications of RFLP. RFLP is based on the variation in DNA sequences among individuals, which affects the **restriction sites** where enzymes cut the DNA. This results in fragments of different lengths that can be separated by **gel electrophoresis**. The technique is widely used for genetic fingerprinting, paternity testing, and evolutionary studies.

## Why Each Wrong Option is Incorrect
- **Option A:** Without specific details on what option A states, it's challenging to provide a direct explanation. However, if option A suggests that RFLP is primarily used for gene expression analysis, it would be incorrect because RFLP is mainly used for analyzing DNA sequences and variations.
- **Option B:** Similarly, if option B proposes that RFLP requires RNA as a starting material, it would be incorrect because RFLP directly works with DNA.
- **Option C:** If option C mentions that RFLP involves PCR (Polymerase Chain Reaction) as a necessary step, it might be misleading. While PCR can be used to amplify DNA before RFLP analysis, it's not a defining characteristic of the RFLP technique itself.

## Clinical Pearl / High-Yield Fact
A key point to remember is that RFLP analysis can be used to diagnose genetic disorders by identifying mutations that alter restriction sites. For example, **Sickle Cell Disease** can be diagnosed using RFLP because the mutation that causes the disease also abolishes a restriction site for the enzyme **MstII**.

## Correct Answer: D.