The correct answer is A) dredging with an existing obstruction. The shapes shown in illustration D018RR depict the signals required for a vessel engaged in dredging or underwater operations with an obstruction present. According to the International and Inland Navigation Rules, vessels engaged in these activities must display two black balls or shapes vertically to indicate their restricted ability to maneuver. This distinguishes them from other vessels not under command or towing, which have different signaling requirements. The other options are incorrect because they do not accurately describe the signaling requirements for the situation depicted in the illustration.