Which number, if typed at an Inmarsat-B SES by a vessel in the Western Atlantic, would result in placing an automatic TELEX call to a subscriber number of 123456?

Correct Answer: Option C — 00230123456+

To place an automatic TELEX call via Southbury (access code 001), the operator dials the code followed by the country code (230 for USA) and the subscriber number, terminated by the + sign. This sequence correctly routes the TELEX through the US gateway to the specified domestic subscriber.