Which of the following is good practice when choosing a transmitting frequency to answer a station calling CQ using FT8?
Why FT8 runs on strict 15-second time slots, and stations alternate: if the CQing station transmits in the even slots, your reply goes out in the odd slots. Since your signal only shares the band with other stations transmitting in your slot, the clear spot you need is one that is quiet during the alternate slot, not during the CQ. Watching the waterfall while the calling station transmits tells you nothing about who will be on that audio frequency when it is your turn.
Watch out Looking for a clear frequency during the same slot as the caller checks the wrong half of the cycle, and it is the mirror-image trap answer; that traffic is not what you will be competing with. Calling on the station's own frequency is an option some operators use, but it is not the practice the question is asking about.
FT8 ping-pongs every 15 s: you transmit in the OTHER slot, so hunt for a clear spot in that other slot.