The top plate.
A stack is a way of organizing data in memory. Data items are visualized as behaving like a stack of physical items. Often a stack is visualized as behaving like a stack of dinner plates. Data are added and removed from the stack data structure in a way analogous to placing plates on the stack of plates and removing them.
Normally with a stack of dinner plates all operations take place at the top of the stack. If you need a plate, you take the one at the top of the stack. If you add a plate to the stack, you place it on top of the stack.
Which plate is the last one that was added to the stack? Which plate will be the first one removed?