Answer:
To tell your cousin about a soccer goal that you made
Step-by-step explanation:
O to express your opinion about a proposed law: If you write a friendly informal letter, you would probably be disgraced and your opinion ignored.
O to thank your teacher for providing encouragement: You should still write a pretty formal letter, if you write one at all, since it is your teacher.
O to tell your cousin about a soccer goal that you made: You can write an informal, friendly letter since it is a cousin, and it is only about a soccer goal you made.
O to ask the mayor if you may interview her for the school paper: If you write a friendly letter to the mayor, you will have about 0% of her saying yes.