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The correct response indicates that vapor service cylinders under 40 lb propane capacity must be equipped with an overfilling prevention device (OPD). This requirement is in place to enhance safety and mitigate the risk of overfilling, which can lead to dangerous situations such as excessive pressure buildup or accidental propane leaks.

The OPDs are designed specifically to prevent the filling of the cylinder beyond the safe level, ensuring that there is adequate vapor space for the gas when it expands due to temperature changes. This feature is especially critical for smaller propane cylinders, as they are often used in portable applications like barbeques or heaters, where safety is paramount.

Understanding the context of the other choices clarifies why they do not meet the criteria for requiring an OPD. Motor fuel cylinders over 40 lb, for instance, typically have different safety standards and regulations. Vapor service cylinders under 40 lb without OPDs could pose safety risks but are not mandated by regulations in the same way as those with OPDs, making option C the most accurate choice related to current safety standards in propane cylinder usage.