"SITOR" communications are based on a digital code system:

Correct Answer: Option B — Consisting of 7 bits, with four zeros and three ones.

SITOR/NBDP uses a 7-bit constant-ratio code defined by CCIR Recommendation 476. Each character is represented by exactly four ones (marks) and three zeros (spaces). This specific ratio allows the receiving equipment to detect errors if the received signal does not maintain this exact bit distribution.