What distinguishes a tube from a pipe in regards to wall thickness?

The choice that states the listed size corresponds to the outside diameter is correct because it reflects a fundamental distinction between tubes and pipes, particularly in terms of how their sizes are defined and measured.

When referring to tubes, the nominal size is often based on the external dimensions, which allows for a standardized approach to ensure consistent sizing across different manufacturers and applications. Tubes are generally used in applications where precise dimensions and the mechanical properties of the material are critical, and understanding the outside diameter is essential for fittings, connections, and structural integrity.

In contrast, pipes are typically sized according to their internal diameter, which serves a different purpose related to fluid flow. This distinction in measurement helps engineers and installers make informed decisions about which material to use based on the requirements of the system they are designing or using.

The other choices do not accurately capture the critical difference in wall thickness and size measurement that separates pipes from tubes.