**Core Concept:** The correct answer is related to the technique used to differentiate the chromosome structure in normal and cancer cells. In this context, chromosomal abnormalities are a hallmark of cancer cells and can be analyzed using specific tests.

**Why the Correct Answer is Right:** Fluorescence in situ hybridization (FISH) is a molecular cytogenetic technique used to visualize and analyze chromosomes, particularly in the context of cancer. It involves the use of fluorescently labeled DNA probes that bind to specific DNA sequences on chromosomes, allowing the identification and quantification of chromosomal abnormalities.

**Why Each Wrong Option is Incorrect:**

A. **False Colors**: Chromosome painting is another cytogenetic technique that uses fluorescently labeled DNA probes to stain and visualize chromosomes. However, this option is incorrect because it does not specifically differentiate between normal and cancer cells.

B. **Chromosome Preparation**: Chromosome preparation, also known as Giemsa staining or banding, is a method to visualize and count chromosomes in cells. Although it is a fundamental technique in cytogenetics, it is not specific enough to differentiate between normal and cancer cells.

C. **Chromosome Analysis**: Chromosome analysis is a broader term encompassing various cytogenetic techniques, including chromosome painting and banding. While it plays a crucial role in studying chromosomes, it doesn't specifically differentiate between normal and cancer cells.

D. **Karyotype**: A karyotype is a detailed description of an organism's chromosome complement. While it provides valuable information about an individual's chromosomes, it is not specific enough to differentiate between normal and cancer cells.

**Clinical Pearl:** Fluorescence in situ hybridization (FISH) is a specific and sensitive method to identify and quantify chromosomal abnormalities, allowing for differentiation between normal and cancer cells. This technique is crucial in the diagnosis, prognosis, and monitoring of cancer treatment effectiveness.

**Correct Answer:** D. Karyotyping

**Why D is Right:** Karyotyping, also known as chromosome analysis, is a comprehensive approach that involves examining the structure and number of chromosomes in cells. By comparing the karyotype of a patient's cells with the normal karyotype, clinicians can identify any structural or numerical abnormalities, which are common in cancer cells. This information aids in diagnosing, predicting prognosis, and evaluating treatment efficacy for cancer patients.

**Why the Other Options are Incorrect:** The other options are incorrect because they refer to specific techniques (Fluorescence in situ hybridization, chromosome painting, banding, and chromosome analysis) or describe a broader concept (karyotype) rather than a method for differentiating between normal and cancer cells. Karyotyping is the appropriate method for identifying chromosomal abnormalities, thus helping to differentiate between normal and cancer cells.