Step 1: Test to apply
Ask: if the system goes from state 1 to state 2 by two different routes, is the change the same?
Step 2: Apply it
Changes in volume, pressure and temperature are the same by any route, since they are fixed by the end states.
Step 3: Work case
Work equals the area under the $P$-$V$ curve, and different curves joining the same two points enclose different areas. So work changes with the route.
Step 4: Result
Work fails the test, so it is the path function. The other three pass the test and are state functions.
Final Answer:
Work is the path function. This is option (C).
\[ \boxed{\text{(C) }\text{Work}} \]