Which of the following refers to the average number of vessels arriving over three years in the detention ratio calculation?

The average number of vessels arriving over a designated period is a critical component in calculating the detention ratio, which is a measure used to evaluate the performance of a ship's safety and compliance record. In this context, the option that correctly identifies this is the company average vessel arrivals.

This measurement reflects the total number of vessels that a particular company has brought into port over a specific timeframe, here indicated as three years. By using the average number of vessel arrivals, authorities can assess how often a shipping company's vessels are detained compared to the total traffic they generate, leading to a clearer understanding of operational compliance and safety awareness within that company.

In contrast, total vessel departures and detention vessel arrivals do not provide relevant information for the average calculation as defined by port state control practices, and average vessel arrivals alone do not specifically attribute the vessel traffic to the permissions of a company. Therefore, focusing on the company average vessel arrivals gives a clear picture for detention ratio calculations, enabling better oversight and enforcement of maritime regulations.