Which type of anti-freeze is toxic and not allowed for cooling potable water?

Ethylene glycol is the correct answer because it is a toxic substance that can have harmful effects on humans and animals if ingested. Its use in systems that supply potable water or are used for drinking is strictly prohibited due to the significant health risks associated with exposure to ethylene glycol. This substance can lead to serious medical conditions such as kidney damage and metabolic acidosis when consumed.

In contrast, propylene glycol is often considered a safer alternative for use in systems that could come into contact with potable water because it has low toxicity and is generally recognized as safe for food applications. Calcium chloride and potassium acetate, while also used in various applications, do not pose the same level of toxicity or health hazards as ethylene glycol when it comes to water systems intended for human consumption. This is why ethylene glycol specifically stands out as the anti-freeze type that is not allowed in these contexts.