"Est-ce que tu aimes le riz ? Oui, j'aime le riz."
I am concerned that this programme sometimes lets you drop the article in translation when "le" is used as in this case, and sometimes does not - according to an answer I had earlier in this lesson for a sentence something like "Do you eat chicken?" where I wrote "le poulet" and was told the answer was "du poulet". Is there a rule?
Yes, there are rules. The first one is that French uses articles nearly everywhere, while English skips them very often.
When the English reads "chicken" and that it means "chicken in general" or "some chicken", the French should have an article, as follows :
I love chicken (in general) = j'adore le poulet (in general) I want (some) chicken (a piece of) = je veux du poulet = "de" (to mean "some") + "le" -> combined in "du".
I love soup (in general) = j'adore la soupe (in general) I want (some) soup (a portion of) = je veux de la soupe = "de" (to mean "some") + "la" -> no combination.
I love children (in general) = j'adore les enfants (in general) I want (some) children ( an undefined number of) = je veux des enfants = "de" (to mean "an undefined number of") + les -> combined in "des".
I love children (in general) = j'aime les enfants
I love these children = j'aime ces enfants
We have only one word for like/love but there is generally no ambiguity. When we "aimons" someone (as opposed to something), it means that we "love" someone.
Note: to be in love with someone = être amoureux /amoureuse de quelqu'un
If we like someone (friendship, sympathy - no family and no sex), we say "je l'aime bien, je l'aime beaucoup".
When it comes to non-human objects, we use a variety of adverbs to indicate the degree of appreciation, or we use to "adorer": "j'adore me promener sous la pluie, j'aime énormément le chocolat..."
Est-ce que= Is it that? = third person singular. Est and ce agree with each other.
Tu manges = you eat = second person singular. Tu and manges agree with each other.
It's hard to be sure if this answers your question because all we can see is the correct answer given at the top of the page. You say that Duo used the tu form es but it does not do so in what we can see. You say that you used the tu form es and it was marked wrong. Where ever you used es in this sentence, it would not be correct.