To unlock the "Teenage Angst" trophy in Life is Strange 2 - Episode 2: Rules, you need to find the fifth optional collectible at the Christmas Market.
Once you enter the Christmas market, head forward towards the fake snowman and then turn left. You'll reach some picnic benches. Turn left again and you'll see a singer in the corner of the sitting area.
You need to interact with her a bit. Give her money on her paper plate. Then select Listen on the singer herself and she'll sing a song. You need to listen to the song in its entirety. Once she's done singing, she'll put her guitar down. Talk to her again and there'll be a lengthy conversation cutscene to get through. Once the girl has left the area, look back towards where she was sitting and look at the guitar pick under the tree. You'll then be able to pick it up as the fifth collectible.