Static methods (and properties, fields, etc) are not instance related - they are "global" to the containing class instead of being involved with a specific chunk of memory.
If you compare it to a car, "drive" is a "static method" for a car, because the instructions apply regardless of which car you are in. Whereas "fill with fuel" is dependant on the specific car - because it could require Petrol or Diesel and only by reference to the specific instance of a car your are trying to fill can you tell which.
So in a static method, you can't refer to instance properties - and that is what
this
is all about: accessing the instance of teh containing class.
Either remove the
static
keyword from the method declaration, or stop trying to use instance related items!
In this case, Application is not instance related, so you shouldn't need the
this
at all:
Word.Range rng = Application.ActiveDocument.Words.Last;
should fix it.