Love apples. I'd like them better in color I think. Oh, sorry, I was just answering your title questions. I don't know, something about this just doesn't feel right. The desat isn't adding anything IMO. The composition also feels awkward, like there's too much space and the red apple needs to be more off-center. You did do a nice job in using selection tools to isolate the single red apple as the editing is not noticeable. All of the above is JMO of course. Good luck in the challenge.
Not getting why you only left one in color? I usually see selective desat as a way to convey a message or make something important stand out. To m it would make more sense if you left a fruit in color that was different ffrom the apples or something like that.