The correct answer is D) joining two lines of equal size. The square knot is a fundamental knot used to join two ropes of equal size or diameter. It is a simple and secure way to connect two lines, making it a common and useful knot for mariners and sailors. The square knot is preferred for joining lines of equal size because it creates a symmetrical and balanced connection, distributing the load evenly across both lines. The other answer choices are incorrect because: A) a different knot, such as the constrictor knot, would be used to prevent a line from unraveling or fraying; B) the sheet bend knot is more appropriate for joining lines of different sizes; and C) the bowline knot is commonly used to form a temporary eye or loop in the end of a line.