Which one of the following is a valid ship station SELCAL?

A valid ship station SELCAL (Selective Calling) consists of a four-character code that is unique to each ship. This code is typically represented as a combination of letters and numbers, with letters often denoting the specific designation of the ship.

The correct answer, which is a four-character code, adheres to SELCAL's format requirements. It represents a valid registration for a specific ship, allowing for selective calling between the ship and control stations.

In this context, the other options do not fit the standard SELCAL format. The first option presents a three-digit number, which does not meet the criteria for SELCAL. The second option, while also a series of digits, is composed of seven digits, which exceeds the required length for a SELCAL code. The fourth option is even longer, containing ten digits, thereby disqualifying it from being a valid SELCAL code.

Thus, the distinguishing feature of the correct answer is its compliance with the established format for SELCAL, making it suitable for selective calling in maritime communication.