Only tiles for the main answer are shown. And then some random noise words. The main answer (shown at the top) does not contain "o'clock". Therefore, the most likely explanation is that the computer chose "'clock" as one of the random words. Your only option then is to just leave it there unused.
In the web version of Duolingo, there's a button under most exercises that lets you toggle between "Use keyboard" and "Use word bank".
The phone app used to have a similar function, but now I see it's gone - instead, it forces you to tap word-tiles in some exercises and type in others...