Which is the code used for SITOR-A and -B transmissions?

The correct answer describes the character structure used for SITOR-A and SITOR-B transmissions, which are part of the SITOR (Simplex Telex Over Radio) system. In this system, each character is indeed encoded using 7 bits, with a binary composition of 4 zeros and 3 ones. This 7-bit encoding is consistent with a modified version of ASCII used in radio communications, allowing for efficient transmission of alphanumeric data in a compact format suitable for radio telex operations.

The choice of 7 bits, particularly with a balance of zeros and ones, is critical for ensuring integrity in data transmission and minimizing the risk of errors. It allows enough combinations to represent the necessary characters while maintaining a level of redundancy that helps in error detection.

The other options do not accurately reflect the bit structure of SITOR transmissions. For instance, 6 bits would limit the character set and make it impractical for the required transmissions, while 8 bits introduces unnecessary complexity for the SITOR system's needs. Lastly, having 5 bits with all "ones" would not fit the character encoding requirements, as it would not provide the necessary variability to represent different data effectively.