**Core Concept**
The question tests the understanding of managing a patient with an **implantable cardioverter-defibrillator (ICD)**, specifically in the context of assessing the device's position and integrity. This requires knowledge of **cardiovascular device management** and **emergency medicine**.

**Why the Correct Answer is Right**
Given the sudden development of shock in a patient with an ICD, it is crucial to assess the device's function and positioning to ensure it is not malfunctioning or misplaced, which could be life-threatening. The best investigation to determine the position and integrity of the ICD would involve imaging that can clearly show the device's placement and any potential issues, such as lead dislodgement.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not directly address the need to visualize the ICD's position and integrity.
**Option B:** Similarly, this option does not provide the necessary information about the device's placement or functioning.
**Option C:** This choice is also incorrect as it lacks specificity in assessing the ICD's position and integrity.

**Clinical Pearl / High-Yield Fact**
In patients with ICDs, any sudden change in clinical status, especially shock, warrants immediate assessment of the device to rule out malfunction. **Chest X-ray** is a critical initial step to evaluate the position and integrity of the ICD, including the leads.

**Correct Answer:** D. Chest X-ray.