Explanation: SITOR-A and SITOR-B (also known as AMTOR for Amateur Teleprinting Over Radio) utilize a specialized 7-bit code for robust digital communication, primarily designed for error detection and correction. The correct code for SITOR transmissions is one where each character consists of 7 bits, specifically with 4 "zeros" (space bits) and 3 "ones" (mark bits). This is known as a constant ratio or constant weight code. This precise 4:3 ratio is crucial for error detection. If a receiver detects a 7-bit character that does not have exactly 4 zeros and 3 ones, it immediately knows the character is corrupted and requests a retransmission (in ARQ mode) or discards it (in FEC mode). Option A is incorrect because the basic Baudot telex code is 5 bits, while SITOR expands upon this with a 7-bit character set to add error detection capabilities. Option B is incorrect because it reverses the correct 4:3 ratio of zeros to ones for SITOR. Option D is incorrect as it describes a different error correction scheme; SITOR's error detection is built into the constant ratio of its 7-bit character, not by appending separate error correction bits to a data character.