Yes actually i did. I just skipped the yes part because it wasn't the subject of the problem. If you write the sentence in singular form, it will be a non correct answer by duolingo but the truth in Indo if its not specified it can be both form.
A generalization would be plural or there would be more information, such as “I like an apple now and then.” which is not correct for this sentence. You could like a specific apple and then it would have been “Yes, I like the apple.”
Either can work here. The English is more commonly plural in this situation. "an apple" could also be possible. Although, “the apple” is more common after this particular verb if it is not the generalization of what someone likes.