I am a native English speaker and I have never used "celeries". I looked it up in my Webster's English Dictionary and celeries is the plural of celery but there is no indication in this sentence that there is more than one celery. I too was maked wrong with my answer, "Who likes celery?" Please add "Who likes celery?" as a correct and most likely answer as it is the common use here.
yang in this context adds emphasis onto siapa. It would be similar to stressing "who" when expressing that no one likes something.