Formal
When writing, there are two types of language—informal and formal. Informal writing will contain elements such as slang, texting lingo/abbreviations, contractions, and figures of speech. Formal language, on the other hand, will not. Formal language will take on what can be considered a professional tone, and formal language should be used when communicating in situations serious/professional in nature. As such, when we look at the example communication you have listed here, we see that the communication is a thank you letter for a job interview. Because a job interview is professional/serious, and because there are no contractions, slang words/phrases, idioms, it can be safely said that the type of language used in this letter is formal.
