For AVL trees, how do we know if a RL or a LR rotation is needed?
In this case the tree is right-left unbalanced. That's because when you insert $B$ it causes the root node to become unbalanced. You can tell its right-left unbalanced from the path that goes from the newly inserted node to the root node i.e. $B$ is the right child of $A$ and $A$ is the left child of $C$. Therefore, it needs a left-right rotation.