Explanation: SITOR ARQ (Automatic Repeat reQuest) is an error-correcting mode primarily used for point-to-point communications. In this mode, the Information Sending Station (ISS) transmits data in blocks, typically three characters at a time. The Information Receiving Station (IRS) checks each received block for errors, often using parity bits within the block. If the IRS detects an error, it automatically requests the ISS to retransmit that specific block. This process ensures extremely high data integrity. Option A describes SITOR's FEC (Forward Error Correction) mode, where characters are sent twice for redundancy in one-way transmissions, not ARQ. Option B is partially correct but less precise and complete than C in describing the specific roles and error checking mechanism. Option D is incorrect because broadcasts, like Maritime Safety Information, are one-way and do not use ARQ, which requires a two-way link for retransmission requests; instead, they utilize FEC mode.