I wrote " I like an apple" should be okay in this translation there is no mistake.
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.”