**Core Concept**
Dysphagia for solids is a clinical symptom that requires a thorough diagnostic evaluation to identify the underlying cause. The primary goal of investigation is to visualize the esophagus and its surrounding structures to identify any abnormalities that may be contributing to the patient's symptoms.

**Why the Correct Answer is Right**
A CT scan is the investigation of choice for dysphagia for solids because it provides high-resolution images of the esophagus and surrounding tissues. This allows for the detection of extraluminal compressions, such as from a tumor or lymphadenopathy, which may be causing the dysphagia. The CT scan can also identify any abnormalities in the esophageal wall, such as a stricture or a mass lesion, which may be contributing to the patient's symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** A barium swallow is primarily useful for visualizing the luminal narrowing or stricture within the esophagus but may not detect extraluminal compressions or abnormalities outside the esophagus.

**Option B:** Endoscopy is useful for visualizing the inside of the esophagus and taking biopsies, but it may not provide sufficient information about the surrounding tissues or extraluminal compressions.

**Option C:** An X-ray chest may show indirect signs of dysphagia, such as aspiration pneumonia or esophageal dilatation, but it is not a specific investigation for dysphagia for solids.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with dysphagia for solids, it is essential to consider both luminal and extraluminal causes, and a CT scan is the best investigation to provide a comprehensive view of the esophagus and surrounding tissues.

**✓ Correct Answer: D. C.T. scan**