Answer: C. Somewhat formal.
Explanation: when writing a letter it is very important to have in consideration the level of formality we want or need to use, it has to do with the person or persons we are writing for (their level of education, position in a job, age, etc), it also has to do with the subject we want to address on the letter, and the context of that subject. If you want to write a letter seeking information for a school club, probable the person you are writing to, shares your age and level of education, that's why it isn't necessary a high level of formality, but it has to be a polite letter, so the correct option is "somewhat formal".
