What is referred to as the direction of travel line on the compass?

The correct choice is the lubber line, which is a critical component of a compass. The lubber line represents the reference mark on the compass that indicates the direction in which the vessel or diver is pointing. When using a compass, the lubber line is used to align the compass with the intended direction of travel, providing a clear visual of where the diver or boat is heading.

Understanding the lubber line is essential for navigation as it helps maintain the correct bearing during a dive or when moving through the water. This marks the direction that the diver should follow in order to reach their desired destination accurately. It is distinct from the other components, such as the north marker, which identifies true north, and the heading line, which is a general term that can refer to various navigation lines but does not specifically equate to the functional purpose of the lubber line.