A signal indicating Greenwich Mean Time would be preceded with what?

The correct answer is identifying that Greenwich Mean Time (GMT) is indicated by the letter "Z." This designation comes from the Zulu time zone in aviation and military contexts, where "Zulu" refers to GMT, which has no offset from Coordinated Universal Time (UTC). In international communication and navigation, the use of "Z" helps ensure clarity and consistency across different time zones, significantly aiding in the transmission of time-sensitive information, such as in weather reports or navigational messages.

The other options do not represent Greenwich Mean Time in the same established way. While the letter "G" may refer informally to time in certain contexts, it lacks the universal acceptance that "Z" has in both military and civil applications. The letter "T" could denote local time in some contexts but is not a recognized standard for GMT. Lastly, "A" does not have any established relationship with GMT and could refer to various other concepts unrelated to timekeeping. Thus, the use of "Z" is crucial for clear and accurate communication regarding time on a global scale.