The correct answer is C) a tug comes alongside while the tanker is loading grade D and E cargoes. The reasoning is that under the regulations, cargo transfer operations on a tank vessel must be stopped when there is a fire on the dock or a nearby vessel (A), or when there is an electrical storm in the vicinity (B), as these situations pose a safety risk. However, the arrival of a tug alongside the tanker does not, in itself, require the cargo transfer operations to be stopped, as long as the tanker is loading grade D and E cargoes, which are less hazardous. The other options, A and B, are incorrect because they do describe situations that would require the cargo transfer operations to be stopped for safety reasons.