In the North Sea area, you sight a buoy showing a quick white light showing 6 flashes followed by one long flash at 15 second intervals. Which of the four topmarks in illustration D031NG below would be fitted to this buoy?

Correct Answer: Option C — C

A light showing 6 flashes followed by one long flash at 15-second intervals is the characteristic of a South Cardinal mark under IALA systems. The corresponding topmark for a South Cardinal mark consists of two cones pointing downward, which is represented by illustration C in the standard IALA buoyage diagrams.