What type of digital code system is used in SITOR (NBDP) communications?

The correct type of digital code system used in SITOR (Selective Information Telex Over Radio), which is also known as NBDP (Narrow Band Direct Printing) communications, operates using a 7-bit code structure. This structure features a combination of four zeros and three ones, which is key to how the data is represented and transmitted.

This 7-bit coding system is part of the standard employed to facilitate effective communication over radio frequencies, allowing for various functions including the transmission of messages and data in a reliable manner. It is critical in maritime communication systems, particularly under the Global Maritime Distress and Safety System (GMDSS), ensuring that vessels can communicate essential information in a standardized format.

The coding system's design, particularly the ratio of zeros to ones, critically impacts how messages are sent and understood, making it essential for effective operation in SITOR communications. The other options do not align with the established protocols used for this system.