What does the context variable Trigger.old represent in an update operation?

Answer is : The previous version values of the record firing the trigger.

Map is returned by trigger.oldMap and List is returned by trigger.old.

I agree. map is not an answer, it should be list. Trigger.newMap eturns the id to record map.

Trigger.Old retunrs list of records and not Map.

I think its D. Previous version values of records firing the trigger