We observed the who/whom war among the native speakers for a long time, and "who" has won, sorry. Some people say "who" is completely wrong, some say that no one uses "whom" in everyday language... would be nice if English could decide.
Anyway, "Whom do they like?" should have been accepted as well, it's on our list of accepted answers.
For the future, if you see an answer that you really feel should have been accepted but the program rejected, it would be good to save a screenshot. Sometimes something was simply forgotten, but often a moderator, like me, claims that the answer should have already been accepted. If you post a screenshot then, we have at least some chance to investigate.
Thank you for your kind words :)
In my App it seems that I can't choose the underlined translations, but there are words to choose and 'whom' is not among them. That was confusing because it is definitely 'whom' and not 'who' as I can remember from the English lessons back in Germany nearly 50 years ago. But in American English it's probably 'who' (makes me think of a great song (Who do you love - Quicksilver Messenger Service)