Final answer:
The worst-case time complexity of the CONTAINS operation in a vanilla Binary Search Tree containing N keys is O(N), which happens when the tree is unbalanced.
Step-by-step explanation:
The worst-case time complexity of the CONTAINS operation in a Binary Search Tree (vanilla BST) containing N keys is O(N). This situation occurs when the tree becomes unbalanced and takes the form of a linear chain (degenerates to a linked list), with each key having only one child. In such a case, the search operation would have to traverse from the root node to the deepest node, leading to a time complexity that is linear with respect to the number of nodes in the tree.