The correct answer is A) square knot. The square knot, also known as the reef knot, is the knot indicated by the letter "W" in the illustration D030DG. The square knot is a fundamental knot used to join two ropes of equal diameter, and is a common requirement in US Coast Guard Captain's License Examinations. The other answer choices are incorrect because: B) clove hitch is used to attach a rope to a cylindrical object, C) barrel hitch is used to attach a rope to a spar or pole, and D) stopper knot is used to prevent a rope from passing through an opening.