**Core Concept**
Intestinal malabsorption refers to the impaired absorption of nutrients by the small intestine, which can lead to various clinical manifestations such as diarrhea, weight loss, and deficiencies of essential nutrients. The diagnosis of intestinal malabsorption involves a combination of clinical evaluation, laboratory tests, and specialized investigations.

**Why the Correct Answer is Right**
The d-xylose absorption test is a suitable test for diagnosing intestinal malabsorption. This test involves the oral administration of d-xylose, a simple sugar that is absorbed by the small intestine. The absorption of d-xylose is then measured in the urine or blood. A low level of d-xylose absorption indicates impaired intestinal absorption, which is a hallmark of intestinal malabsorption. The d-xylose absorption test is particularly useful for evaluating the absorptive capacity of the small intestine, especially in cases where other tests are inconclusive.

**Why Each Wrong Option is Incorrect**
**Option A:** The Schilling test is used to assess the absorption of vitamin B12, which is primarily absorbed in the terminal ileum. While the Schilling test can provide information about the absorption of vitamin B12, it is not a direct test of intestinal malabsorption.

**Option C:** The lactose tolerance test is used to diagnose lactose intolerance, a specific type of malabsorption caused by the deficiency of lactase enzyme in the small intestine. While lactose intolerance is a form of malabsorption, the lactose tolerance test is not a suitable test for diagnosing general intestinal malabsorption.

**Clinical Pearl / High-Yield Fact**
The d-xylose absorption test is a useful diagnostic tool for intestinal malabsorption, but it is essential to note that a low result may indicate not only intestinal malabsorption but also other conditions such as pancreatic insufficiency or small bowel bacterial overgrowth.

**Correct Answer:** C.