All of the following can lead to the chemical contamination of food except:

The option regarding using a backflow prevention device on a carbonated beverage dispenser is correct because this practice is designed to prevent contamination rather than cause it. Backflow prevention devices ensure that contaminated water does not flow back into the potable water supply, thereby protecting the safety and integrity of beverages served to consumers. This device is a critical component in maintaining food safety and preventing chemical or biological contaminants from entering the food and drink supply.

In contrast, the other choices potentially lead to chemical contamination. Cooking acidic foods, like tomato sauce, in copper pots can result in the leaching of copper into the food, which is harmful when ingested. Storing orange juice in a tin container might cause chemical reaction where the acidity of the juice interacts with the tin, leading to a metallic taste and potential toxicity. Serving fruit punch in a galvanized tub can introduce zinc from the galvanized coating into the drink, which can be unsafe in high quantities. Each of these scenarios poses a risk for chemical contamination, reinforcing the value of using safe food storage and preparation practices that are free from harmful materials.