Most people think that ulcers are caused by psychological stress. it was discovered that an acid-resistant bacterium, heliobacter pylori, lives in the lining of the stomach. if these bacteria cause ulcers, then treatment with an antibiotic should reduce ulcers. this line of investigation is an example of

Hypothetico-deductive reasoning.

Explanation:

Hypothetico-deductive reasoning may be defined as the scientific process of the producing different hypothesis on a particular topic. One of the hypothesis can be taken of the particular topic and be tested scientifically.

Some people think that psychological stress can cause the ulcers, it is a type of scientific hypothesis. This hypothesis was further tested and later found that ulcers occur due to acid resistant bacteria. This investigation correctly determines the hypothetico-deductive reasoning.

Thus, the answer is hypothetico-deductive reasoning.