Which one of the following is a grammatically correct sentence?

Option D is the correct answer.

Option D uses the correct grammatical structures to form the sentence.

Option A: This is not correct because the verb "waiting" requires the preposition "for" when followed by an object like "you." The correct form would be "I have been waiting for you for an hour."

Option B and Option C: Both are incorrect because the article "a" should be "an" before a word starting with a vowel sound, like "hour" . The correct form would be "I have been awaiting you for an hour."