**Core Concept**
Zollinger-Ellison syndrome (ZES) is a rare disorder characterized by excessive gastric acid secretion due to high levels of gastrin, a hormone that stimulates gastric acid production. This condition is often caused by a gastrin-secreting tumor, typically a gastrinoma, which can be located in the pancreas or duodenum.

**Why the Correct Answer is Right**
The most important investigation for diagnosing ZES is the secretin stimulation test. This test involves administering secretin, a hormone that normally inhibits gastrin release, and measuring the resulting gastrin levels. In patients with ZES, the gastrin levels will paradoxically increase in response to secretin, due to the tumor's autonomous secretion of gastrin. This is because the gastrinoma is not responsive to the normal regulatory mechanisms that control gastrin release.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastrin levels are elevated in ZES, but measuring basal gastrin levels alone is not sufficient for diagnosis, as other conditions can also cause elevated gastrin levels.
**Option B:** Endoscopy is useful for visualizing the stomach and duodenum, but it is not the most sensitive test for diagnosing ZES.
**Option C:** Ultrasound and CT scans can help identify the location of the gastrinoma, but they are not as specific as the secretin stimulation test for diagnosing ZES.

**Clinical Pearl / High-Yield Fact**
The secretin stimulation test is a key diagnostic tool for ZES, but it requires careful interpretation of the results. A positive test result is defined as a gastrin level increase of at least 200 pg/mL above the baseline level.

**Correct Answer:** C.