It's impossible to know if it's tú or Usted without context, so Duo often adds something to indicate who you're talking to. So like the other comments say, the sir/señor indicates that it's addressing someone politely (like a store employee talking to a customer).
"cuál"' is not followed by a noun. "Cual" can be followed onely by a verb, or the preposition "de".
I recently read a link someone here provided realfastspanish.com, it was very good. Basically, qué is used three ways: to provide a definition, when immediately followed by a noun, or, when followed by a verb, its when your answer is not from a list of choices, like when someone asks, what happened, what do you want to do, or what did you do. Examples: qué es...what is a dog, qué (noun) what dog do you like, or qué (verb)... what did your dog do. Cuál is used for a choice. Cuál es...What is your favorite dog, cuál de...which of your two dogs is taller, or cuál (verb)...which of these three dogs do you like the best.