"Where are you at?" is slang, at least in America, whereas "Where are you?" is the proper way to ask where one is. "Where are you at?" is incorrect, ending a sentence with a preposition is a most grievous sin!

Is at grammatically correct?

So, the word at is a preposition. A preposition is, in a sense, a connector. When you use the word at in a sentence, you should notice that it smoothly connects the other words around it. If the word at isn't working as a connector, then it will most likely be working as an adverb.

Which is grammatically correct sentence?

In order for a sentence to be grammatically correct, the subject and verb must both be singular or plural. In other words, the subject and verb must agree with one another in their tense. If the subject is in plural form, the verb should also be in plur al form (and vice versa).