Step 1: Approach
Picture the shape of each material and ask which direction is thin.
Step 2: Shapes
A thin film is like a sheet of foil: very thin in one direction and wide in the other two. A nanowire is like a thread: thin in two directions but long in one. A quantum dot is a tiny crystal that is small in all three directions. A nanoring is a tiny loop, small in every direction.
Step 3: Count of thin directions
Film: 1. Wire: 2. Quantum dot: 3. Ring: 3.
Step 4: Conclusion
Only the thin film has a single dimension below 100 nm, option (C).
Final Answer:
A thin film is nanoscale only in its thickness, so the answer is option (C).
\[ \boxed{\text{Thin films}} \]