Which of the following synchronizes WSJT-X digital mode transmit/receive timing?

Explanation

WSJT-X modes transmit and receive in fixed periods aligned to UTC, and the software takes the start of each period from the computer's clock, which must be kept within about a second of correct time by NTP or GPS. The signal itself carries synchronization vectors that align the decoder to the symbols and frame, but nothing in it identifies which UTC period a transmission belongs to, so a badly set clock still means no decodes.