**Core Concept**
The patient's presentation of shortness of breath, hypotension, soft heart sounds, elevated JVP, and reduced QRS complex amplitude on ECG is suggestive of cardiac tamponade, a condition characterized by fluid accumulation in the pericardial sac compressing the heart and impeding its function. This clinical scenario requires a diagnostic imaging modality that can evaluate cardiac structure and function, particularly in the context of fluid accumulation.

**Why the Correct Answer is Right**
Two-dimensional (2D) echocardiography is the investigation of choice for diagnosing cardiac tamponade. This non-invasive imaging modality uses ultrasound waves to produce detailed images of the heart's structure and function. It can accurately assess the presence and extent of pericardial effusion, as well as its impact on cardiac function. The Doppler component of 2D echocardiography can also evaluate the hemodynamic effects of tamponade, such as decreased cardiac output and increased filling pressures. The rapid availability and high sensitivity of 2D echocardiography make it an ideal diagnostic tool for this condition.

**Why Each Wrong Option is Incorrect**

* **Option A:** Ultrasound (USG) is not specific enough for diagnosing cardiac tamponade, as it may not provide detailed images of the heart's structure and function.
* **Option B:** Positron emission tomography (PET) is not typically used for diagnosing cardiac tamponade, as it is more commonly employed for evaluating metabolic activity and perfusion in various conditions, such as cancer and cardiovascular disease.
* **Option D:** M-mode echocardiography is a one-dimensional imaging modality that provides a limited view of the heart's structure and function, making it less suitable for diagnosing cardiac tamponade compared to 2D echocardiography.

**Clinical Pearl / High-Yield Fact**
Cardiac tamponade is a medical emergency that requires prompt recognition and treatment. The Beck's triad, consisting of hypotension, muffled heart sounds, and elevated JVP, is a classic clinical presentation of this condition. Early recognition and initiation of treatment, which may include pericardiocentesis or surgical intervention, are critical for improving patient outcomes.

**✓ Correct Answer: C. 2 - D Echocardiography**