Choose the sentence which is most appropriate – grammatically, semantically & logically.

"Sorry to keep you. How long have you been waiting?" is correct: "Sorry to keep you" is idiomatic and "How long have you been waiting?" uses the present perfect continuous with correct question word order. "How long are you waiting?" uses the wrong tense, "How long you have been waiting?" has statement word order in a question, and "Since how long are you waiting?" is non-standard.