The correct answer is D) shorten a line. The sheepshank knot is used to temporarily shorten a line or rope. It is formed by creating two loops in the line and passing the working end through both loops. This allows the length of the line to be adjusted as needed, making it a useful knot for tasks like securing cargo or adjusting the length of a mooring line. The other answer choices are incorrect because: A) the sheepshank does not specifically prevent fraying, B) it is not primarily used to join lines of unequal size, and C) it does not stop off a line (i.e., secure the end of a line).