Koen Verbeeck (10/27/2011)
Nice question, but answer number 4 was also incorrect, as both are of the geometry datatype.
A polygon can also be a geography datatype, but according to MSDN, Linestring is geometry.
If something in MSDN says a geometry value can't be a LineString the (a) MSDN is self contradictory and (b) the bit that says that is just plain wrong. I haven't been able to find such a bit, maybe you are misreading something? The BoL page Types of Spatial Data is pretty clear that a LineString can be a geometry, since it points out the distinction between the two types in how the connecting edges of a LineString are defined.
Try the following code (it works; it wouldn't if geography didn't include linestrings):-
DECLARE @g geography;
SET @g = geography::STGeomFromText('LINESTRING(-122.360 47.656, -122.343 47.656)', 4326);
SELECT @g, @g.STAsText();
Also see Constructing Geography Instances.