"Sir, I have a question."
Translation:Señor, yo tengo una pregunta.
La señora Sánchez - es amable.
El profesor es genial.
When talking about someone and include the title, you also include the definite article. This is just like it would be for a profession. This is quite common in other language; it's just that English doesn't do so for some reason.
Señora Sánchez, usted es amable.
Profesor, usted es genial.
If you're talking to the person directly, you don't use the article, just like you wouldn't do so for a profession.
I'm hazarding a guess as to what you're asking, but I should note that things get easier if you formulate questions as complete sentences.
Why are are they using informal "yo tengo", although the sentence starts with "Señor"? Well, the start merely addresses whom you are talking to, here. Whether this sentence starts with "Filipe VI, king of Spain", with all his titles, or as "Guttersnipe trying to steal my wheel caps", it's just the person you're addressing. After the address, after the comma, you're getting to the sentence itself, and in this case the second person isn't mentioned. (The second person is the one where you have a choice between formal and informal.)
Now, imagine the sentence: Señor, tú dejaste una de mis tapas de ruedas. Now, the main sentence does include a second person, if I didn't mistype it, and you do have to chose how polite you are. In this case, you give him the benefit of the doubt and say "Sir", but then you call him "you" when mentioning that he left one of your wheel caps (implying that he took the others). If you are OK with the transaction, you might have said "usted dejó ...", thus addressing him politely all the way.
The world is more or less round, but I'm not sure why you mention it. I'm also not sure whom you're addressing, hence let me state explicitly that this is a forum. A Duolingo forum is not the way to reach the contributors to a course if that's what you intended; for that, the report feature is better suited. The forum on the other hand is where you can converse with your fellow students.
What it is that your asking about isn't clear to me, as a fellow student. The best way to ask in a forum what is going wrong is to copy the question and your answer(s) here, and give your reasoning of why it would be correct, etc. We fellow students might be able to help you out, but of course we're limited in what we know, so the more information you give, the better the chance that at least one of us can figure out the problem.
That's actually a good example of why the "also acceptable answers" can be so different from what we already know. If someone adds "duda" as something that would also be possible, in some situations Duolingo would give duda as alternative when you make a mistake, because it simply picks a correct answer, not necessarily a mythical best answer. Thus, now you, who made the mistake, might wonder: 'Why does Duolingo mention "dudar" (hesitate) here?' And the answer would be: For no particular reason; it's just that the editors agreed that in some situations it can be used to introduce a further question.