**Core Concept**
Breast cancer diagnosis often requires a combination of imaging modalities and histopathological examination. The definitive investigation for carcinoma breast is crucial for accurate staging and planning treatment.

**Why the Correct Answer is Right**
Mammography, ultrasound, and magnetic resonance imaging (MRI) are commonly used for breast cancer screening and detection. However, biopsy is the gold standard for diagnosing breast cancer. Fine-needle aspiration cytology (FNAC) is a minimally invasive technique that involves inserting a thin needle into the breast tissue to collect cells for examination. Core needle biopsy (CNB) involves using a larger needle to collect tissue samples for histopathological examination.

**Why Each Wrong Option is Incorrect**
**Option A:** Mammography is an essential tool for breast cancer screening, but it is not the definitive investigation for diagnosing carcinoma breast.
**Option B:** Ultrasound is useful for evaluating breast lesions, especially in young women and those with dense breast tissue, but it is not the definitive investigation for diagnosing carcinoma breast.
**Option C:** MRI is highly sensitive for detecting breast cancer, particularly in women with a high risk of developing the disease, but it is not the definitive investigation for diagnosing carcinoma breast.

**Clinical Pearl / High-Yield Fact**
Fine-needle aspiration cytology (FNAC) and core needle biopsy (CNB) are both effective techniques for diagnosing breast cancer, but CNB provides more accurate results by allowing for histopathological examination of tissue samples.

**Correct Answer:** D. Core needle biopsy (CNB).