What unit of measure is equivalent to 6 feet in sounding lines?

The correct choice is fathom because it is a unit of measurement specifically used in nautical contexts, particularly for depth and sounding lines. One fathom is equal to 6 feet, which makes it particularly applicable for hydrographic surveying where depth measurements of water bodies are crucial. This unit is derived from tasks related to nautical navigation and maritime activities, making it the standard for recording depth underwater.

In contrast, meter, league, and yard are units of measure that have their own standard applications but do not align as closely with the context of sounding lines in hydrographic surveying. A meter is approximately 3.28 feet; a league is a much larger measurement traditionally used for distance on land (about 3 miles); and a yard is equal to 3 feet, which does not accommodate the traditional nautical measurement system. Therefore, fathom is the most relevant and accurate unit of measure for depths in sounding lines.