The correct answer is C) keep a line from passing through a sheave. The figure eight knot is commonly used to create a stopper knot at the end of a line to prevent it from passing through a pulley, block, or sheave. This knot is stronger and more secure than a simple overhand knot for this purpose. The figure eight knot is not typically used to shorten a line or join lines of equal size, which are the functions of other specialized knots. Therefore, answer C correctly identifies the primary use of the figure eight knot in the context of a Captain's License Examination.