Like t123 said, I think the key to this question was to rule out the other answer choices as they would not have normal stool. I found this article that essentially breaks down GI disorders in diabetes into gastroparesis and intestinal enteropathy. These complications and their symptoms are often caused by abnormal GI motility, which is a consequence of diabetic autonomic neuropathy involving the GI tract.
Intestinal enteropathy in patients with diabetes may present as diarrhea, constipation, or fecal incontinence. The prevalence of diarrhea in patients with diabetes is between 4 and 22 percent.4,5 Impaired motility in the small bowel can lead to stasis syndrome, which can result in diarrhea. In addition, hypermotility caused by decreased sympathetic inhibition, pancreatic insufficiency, steatorrhea, and malabsorption of bile salts can further contribute to diarrhea. Abnormal internal and external anal sphincter function caused by neuropathy can lead to fecal incontinence. When evaluating a patient with diabetes who has diarrhea, drug-related causes (e.g., metformin [Glucophage], lactulose) should be considered.
This question is really testing what the different types of diarrhea show. Key is "stool shows no abnormalities". In malabsorption, exudative, osmotic, and secretory diarrhea, your poop would be weird in some way. A motility disorder is the ONLY type here that would cause normal poop.