The correct answer is D) With arm extended and fingers closed, point thumb downward. This is the correct hand signal to instruct a crane operator to lower the boom. This hand signal is commonly used in maritime and construction operations to convey instructions to crane operators in a clear and standardized way. The thumb-down gesture with the arm extended is an unambiguous signal that indicates the desired action of lowering the crane's boom. The other options are incorrect because they do not represent the standard hand signal for lowering the boom. Option A describes a motion that could be interpreted differently, option B does not provide a clear directional signal, and option C does not involve any hand gestures to communicate the desired action to the crane operator.