The answer to your question would be that the sentence that is written using informal language is the following one: Wow, that's an awesome painting. That is, your answer would be A.
The selected sentence contains an interjection (wow). Interjections express meaning or feeling in a word or two. They simply convey the way the author or speaker is feeling. The fact is that interjections are rarely used in academic or formal writing. On other hand, the adjective "awesome" is slang, so it also contributes to the informality of the sentence.