What is `wasted space` parameter and why is it O(n) for a linked list
Different people seem to define "wasted space" in different ways. Some authors define it as any space in a data structure that's not used to store actual data; others define it as space that could be used to store data but isn't being so used at the moment.

So, in the first definition, a linked list has $O(n)$ wasted space because every entry of the list contains some piece of data but also a pointer, so a constant fraction of the space taken up by the data structure is "wasted". (I think this is a silly definition of "waste": the space taken up by the pointers isn't wasted; it's an overhead.)  In the second definition, a half-full hash-table has 50% wasted space (again, $O(n)$).