Final answer:
The PUSH operation in a linked cell stack has a worst-case time complexity of O(1) because it involves adding an element to the top, which is a constant-time operation.
Step-by-step explanation:
When we consider a stack implemented using linked cells containing N elements, the operation of adding an element, known as PUSH, has a worst-case time complexity of O(1). This is because pushing onto a stack involves adding an element to the top, which in a linked list implementation means creating a new cell (or node) and setting its next reference to point at the current top, then updating the top to the new node. This operation does not depend on the number of elements currently in the stack, making it a constant-time operation.