Explanation:

- Base: Bases are not typically associated with causing stomach pain. They are alkaline substances and are usually used to neutralize excess stomach acid.

- Acid: Excess stomach acid can lead to discomfort and pain, commonly associated with conditions like acid reflux or ulcers.

- Salt: Normal salt intake does not cause stomach pain, but excess salt can contribute to issues like high blood pressure or kidney strain, which might indirectly affect overall health.

- Bacterium: Certain bacteria, like Helicobacter pylori, can cause infections in the stomach lining, leading to ulcers and stomach pain.

The correct answer is:

Option 4: Bacterium