You need to say more. Nobody will give you decent marks for just naming a song! You have to explain WHY it's your favourite song. Talk about the words and the melody. Explain what the song is about, how it makes you feel and why it is important or meaningful to you. Relate the song to your life and your experiences, or to what kind of person you are. Tell the examiner about the first time you heard this song, and the impression it made on you. Remember that you don't have to tell the truth, but you do have to be able to express your opinions fully and expand on your initial statement. Why don't you have a think about this, and then write a paragraph in the Notebook section?