My testing on duolingo suggest clitic pronouns like "le" are non-optional. "le" = "a usted" here and "a usted" is the optional part since it tells us for sure it is "you" and not "ella"/"she" or "él"/"he". Think of "a usted" as for clarification only or as redundancy to "le" but the "le" is not optional.
Are you saying that it accepted What has happening to you? Because that isn't good English. I now notice What is happening with you? And now realize that it must be an idiom. Que le pasa a usted? What I've learned from this is the indirect object pronoun must be present but other words can be implied "con" for example.
I've never thought of it as being implied, the way English implies with an eliptical sentence (EX: He is better than I. [implied "am"]). I've always thought of it as the verb actually changing meaning. I like the idea of it being implied rather than a literal change in meaning. By the way, after I started reading these posts, I thought that "What's up with you?" might be a translation that kept both the flavor and the acceptable literal translation in a good balance.
Ok now i am really cornfused. Qué le pasa a él was said to mean what is wrong with him. Now qué le pasa a usted is meaning what is happening with you. Is it just a regional thing between the two different meanings? I get Qué pasa- whats happening, Qué le pasa- whats happening with him/her/it and Qué le pasa a usted. But how can it also be what is wrong?
Qué le pasa a él? can have two meanings - What's happening with him? or What's wrong with him?. The context or situation should dictate to you which one is correct. That's why Duolingo should not mark the translation incorrect if you use the latter meaning as I did with What's the matter with you?.
The drop-down hints are frequently wrong/misleading. The right answer is usually there too, but it's up to you to pick the right one. They're not providing the drop-down hints like "Any of these answers will work" but it's more like a multiple choice test.
I don't use the drop-down hints at all anymore, I either keep a paper dictionary near me if I'm using the phone app, or if I'm online, just use WordReference or SpanishDict instead