Which of the following is an example of an illegal connection?

An illegal connection refers to a situation in gas piping systems that violates safety standards or regulations. Cross connections downstream of the meter are a clear example of this because they can compromise the integrity of the gas supply and pose safety risks. When gas lines are improperly connected, it can lead to contamination or backflow from other systems, creating dangerous conditions in homes or commercial properties.

In contrast, correct meter placement, properly sized fittings, and standard service lines all adhere to established codes and practices that are designed to ensure safe and efficient gas distribution. Meter placement ensures accurate measurement of gas usage, properly sized fittings prevent leaks and ensure a secure connection, and standard service lines provide safe transportation of gas to appliances. Hence, the inclusion of cross connections downstream of the meter is indeed what makes it illegal in the context of gas piping systems.