Final answer:
The average case time complexity of a post-order traversal for a BST with N nodes is O(N), as each node is visited exactly once during the traversal process.
Step-by-step explanation:
The student asked the average case time complexity of a post-order traversal for a Binary Search Tree (BST) with N nodes. The correct answer is a) O(N). Unlike operations such as insertion, deletion, or searching for a particular element, where the average case time complexity is O(log N) due to the binary nature of the decision path in a balanced BST, traversal operations like in-order, pre-order, and post-order traverse every node exactly once to visit all elements of the tree. Therefore, the time complexity is linear with respect to the number of nodes in the tree, irrespective of the tree's structure.