Answer:
The answer is A.
Step-by-step explanation:
For number B, the diction is rather casual and seems like something that would be said in normal conversations.
For number C, it uses a bit of slang by using the word "ya" instead of "you".
For number D, it uses more slang, by using the word "u" instead of "you", and there seem to be some capitalization errors as well.
However, in number A, it is not only written in a formal way with all grammatically correct punctuations but also because of the word usage "thee". "Thee" is not often used in conversations, and it is often used in old-fashioned, poetic, or religious ways. And, since poetics and religions are indications of formal concepts, A would be the sentence that is formally written.
Hope this helped!
~Jinachi~