What imbalance is fixed by a single right rotation?

It balances a tree when a long path exist in the left subtree of the left of the root.

Explanation:

Rotation is an operation which is performed on the binary tree which changes the structure of the binary tree but it does not interfere with the order of elements.

A single right rotation strategy is basically for re balancing the tree. When there is along path which exists in the left subtree of the left of the root.