What is first sent by all MF-HF DSC transmissions?

Correct Answer: Option B — A string of dots to stop the scan receiver.

ITU-R M.493 specifies that MF/HF DSC transmissions begin with a dot pattern (a preamble of alternating B and Y states). This sequence allows scanning receivers to detect the signal, stop scanning, and lock onto the specific frequency before the actual message data begins.