Which type of fitting is acceptable for the introduction of supply or return lines at the top of the tank?

The correct answer involves the use of a compression fitting for the introduction of supply or return lines at the top of the tank. Compression fittings are designed to create tight seals around pipes without the need for welding or other permanent connections. This type of fitting allows for easy installation and provides a reliable leak-proof joint, which is crucial for systems carrying fuel. The ability to remove and replace lines without damaging the tank adds to their practicality and ensures that maintenance can be performed efficiently.

Other types of fittings, such as threaded fittings, may not always provide the same level of leak resistance under pressure and temperature fluctuations. Welded fittings, while strong and durable, create a permanent connection that may complicate repairs or replacements. Capacitor fittings are not relevant in this context, as they are not a recognized type of fitting for fluid transfer. Therefore, the specific characteristics of compression fittings make them the most suitable choice for this application.