You see a red sidelight bearing NW (315). That vessel may be heading _____________.

When observing a red sidelight that bears NW (315 degrees), it indicates the port side of the vessel. In nautical navigation, a red sidelight is always positioned on the left side (port side) of the vessel when it is facing forward. This means that if the sidelight is observed from a stationary position and it appears to bear 315 degrees, the vessel is maneuvering in such a way that its bow is pointing toward the southeast quadrant.

To determine the heading of the vessel, if you imagine the direction that the port side light (red) should be facing, the opposite would be the direction in which the vessel is headed. The vessel is likely to be heading toward the south (180 degrees) as that would put the red sidelight bearing 315 degrees to your left side. Given this understanding, the most logical conclusion is that the vessel is heading south.