What lights would be visible from astern of a 40-meter tug that is pushing two barges ahead?

In the context of a towing vessel such as a 40-meter tug pushing two barges, the navigation lights that would be visible from astern are primarily determined by the regulations outlined in the COLREGs (International Regulations for Preventing Collisions at Sea).

For a tug under power, when pushing ahead, it is required to show a sternlight, which is the light specifically meant to indicate the vessel's presence from behind. This sternlight helps other vessels understand the vessel’s position and direction when they are coming from the aft.

In addition, the tug would display an all-round white light, but this light is typically visible in all directions and is not exclusively for viewing from astern. The two masthead lights would also be visible to those approaching from the front rather than the stern.

Thus, the reason the correct answer identifies the sternlight is that it is the specific navigation light intended to communicate the tug's operation when viewed from behind. This adheres to regulations that stipulate the appropriate use of navigation lights based on the configuration and activities of the vessel to ensure safety and visibility on the water.