Final answer:
The enzyme with an optimum activity at pH 2.5 will be most active in the stomach, as it is the part of the alimentary canal with a matching acidic environment.
Step-by-step explanation:
An enzyme showing optimum activity at a pH of 2.5 will be most active in the part of the alimentary canal that matches this acidic environment. Given the choices provided and considering the nature of gastric digestion, the correct answer is B. Stomach. The stomach is known for its highly acidic environment, with a pH range of 1.5 to 2.5, which is ideal for enzymes like pepsin that break down proteins in food. In contrast, the small intestine, including the duodenum and ileum, has an alkaline environment due to the presence of bicarbonate in the pancreatic juices, making the pH less suitable for an enzyme with an optimum activity at pH 2.5.