## **Core Concept**
The core concept being tested here is the diagnosis of breast cancer. Breast cancer diagnosis involves various methods, including imaging techniques and histopathological examination. The definitive diagnosis of breast cancer requires confirmation of malignant cells.

## **Why the Correct Answer is Right**
The correct answer, **. Histopathological examination**, is the gold standard for diagnosing breast cancer. This method involves examining tissue samples under a microscope to identify cancer cells. It provides detailed information about the type of cancer, its grade, and whether it has spread to lymph nodes or other tissues. This information is crucial for planning treatment.

## **Why Each Wrong Option is Incorrect**
- **Option A: Mammography** is a widely used imaging technique for screening breast cancer, but it is not definitive. It can detect abnormalities suggestive of cancer, but it cannot confirm the diagnosis.
- **Option B: Ultrasound** is another imaging technique used, especially in younger women or to differentiate cystic from solid lesions. However, like mammography, it is not definitive for diagnosing cancer.
- **Option C: FNAC (Fine Needle Aspiration Cytology)** can provide cytological information and is often used for initial assessment. However, it may not always provide sufficient information about the tumor's architecture or invasiveness, making it less definitive than histopathological examination.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while imaging techniques like mammography, ultrasound, and MRI are crucial for detecting breast lesions and assessing their extent, **histopathological examination is the only definitive method for diagnosing breast cancer**. This is because it directly visualizes the cancer cells and their characteristics.

## **Correct Answer: . Histopathological examination**