Short Answer 
The backflow prevention device is the safest option for carbonated beverage dispensers as it does not lead to toxic metal poisoning, unlike materials such as copper, galvanized steel, and pewter that can leach harmful metals into food and drinks.
Step 1: Understand the Risks of Toxic Metals
Toxic metal poisoning can stem from exposure to substances like lead, mercury, copper, and cadmium. Understanding these risks is crucial; analyze each material to see if it poses a threat:
- Copper pots: Can leach copper into acidic foods like tomato sauce.
- Galvanized tubs: Zinc can leach into acidic juices, which may lead to toxicity.
- Pewter pitchers: May contain lead that can leach into citrus juices.
Step 2: Evaluate Each Material
Each material used in food or beverage preparation carries varying risks of toxic metal leaching. Assess them closely:
- Copper pots pose risks primarily with acidic foods.
- Galvanized containers can cause zinc leaching when used for juices.
- Pewter can release lead into drinks, especially acidic ones like orange juice.
Step 3: Identify the Safe Option
After analyzing the materials, it becomes clear that not all equipment is safe. The best choice is:
- Backflow prevention device: Built to keep potable water uncontaminated and constructed from non-toxic materials, ensuring it does not pose poisoning risks.
Consequently, using a backflow prevention device on a carbonated beverage dispenser is the safest option regarding toxic metal poisoning.