Answer:
Correct answer is B.
Step-by-step explanation:
When we write informal letter, we usually write same as we speak, so informal language, contractions and slang words are allowed, such as the use of "cool" in answer B.
When it comes to formal language, contractions (it's, we're, etc.) are not allowed, as well as slang words. Answers A, C and D can be suitable for formal report since they use formal grammar and language with exact details.