Final answer:
Iron supplements can cause gastrointestinal upset due to their harshness on the digestive system, leading to symptoms like constipation and stomach discomfort.
Step-by-step explanation:
The supplement that can cause gastrointestinal upset is iron. This is because iron supplements are known to be harsh on the digestive system and can lead to symptoms such as constipation, nausea, and general stomach discomfort. In contrast, the water-soluble vitamin vitamin C is generally well-tolerated, though excessive doses can also cause gastrointestinal symptoms. To avoid adverse effects, it is important to stick to the recommended dietary allowances or the Tolerable Upper Intake Levels for each supplement.