What is the invariant of the Stack Class $($Linked$-$List Version$)?$

a$.$ The number of items in the stack is stored in the member variable used.

b$.$ The items in the stack are stored in a linked list, with the top of the stack stored at the head node, down to the bottom of the stack at the tail node.

c$.$ The items in the stack are stored in a partially filled array called data, with the bottom of the stack at data$[0],$ the next entry at data$[1],$ and so on to the top of the stack at data$[$used$-1].$

d$.$ The member variable top$\_$ptr is the head pointer of the linked list of items.