Which of the following tree data structures is not a balanced binary tree?

B-trees are indeed not balanced binary trees.

Key difference: While they are balanced, they are not binary trees.

Multiple children: Unlike binary trees, which have at most two children per node, B-trees can have multiple children (typically a minimum number defined by the order of the B-tree).

Efficiency: B-trees are designed for efficient storage and retrieval of large datasets on disk, where reading and writing data from disk is relatively slow. By allowing multiple children per node, B-trees can reduce the number of disk accesses required for search, insert, and delete operations.

In summary, while B-trees maintain balance properties to optimize performance, they deviate from the strict binary structure of binary trees.

The other options:

Splay trees, AVL trees, and Red-black trees are all types of balanced binary trees, meaning they have specific rules to maintain balance and ensure efficient operations.