The "square knot" is primarily used for __________.

The square knot is primarily used for joining two lines of equal size, which is why the answer is considered correct. This knot is particularly useful in scenarios where two ropes or lines of similar diameter need to be securely tied together to form a longer length or to combine strength.

The square knot, also known as a reef knot, is characterized by its simplicity and ease of tying. When tied correctly, it creates a flat and secure join that lies neatly against the surface of the materials being joined. Its effectiveness relies on the equal tension present in both lines, which helps maintain its integrity under load.

In contrast, other knots or tying techniques would be more suitable for situations involving lines of different sizes or specialized tasks, underscoring the specific utility of the square knot for equally sized ropes. The design and intended use of the square knot make it less effective for other options related to temporary eyes or preventing fraying, as those scenarios call for different knot types or techniques altogether.