Final answer:
A 45-year-old man with celiac disease for 12 years is most at risk of developing intestinal lymphoma, which is associated with the chronic inflammatory state caused by the disease.
Step-by-step explanation:
Individuals with celiac disease are at an increased risk of certain types of intestinal malignancies, particularly due to the chronic inflammation and immunological dysregulation associated with the disease.
Among the listed options, the one that a 45-year-old man suffering from celiac disease for the past 12 years is most at risk of developing is intestinal lymphoma. This association has been documented in the medical literature and is linked to the ongoing inflammatory state in the intestine caused by the immune response to gluten.