Why does Duane Sr. claim his son has an iron stomach?

The assertion that Duane Sr. claims his son has an iron stomach is best explained by the idea that the son ate unusual items as a child. This phrase suggests that the son’s past experiences with various types of food, potentially including strange or non-traditional items, has led to a strong tolerance for different flavors and food textures. Eating unusual items often conditions a person's digestive system to handle a range of foods without adverse reactions.

While the other choices mention aspects like eating spicy food, never getting sick, or having the ability to drink any beverage, they do not specifically address the origin of the son's strong stomach as effectively as the idea that he grew up consuming atypical foods. Hence, the claim stands on the premise that childhood experiences with a diverse diet are a key factor in developing such a resilient digestive system.