Step 1: Understand remote sensing energy.
Remote sensing works by recording energy that the Earth's surface reflects or gives off in certain parts of the electromagnetic spectrum.
Step 2: Check the visible region.
The visible region records reflected sunlight and clearly shows surface features, so it is useful.
Step 3: Check the infrared region.
The infrared region records reflected and emitted heat energy, which is very useful for vegetation and temperature studies.
Step 4: Check the microwave region.
The microwave region passes through clouds and is used by radar, so it is useful too.
Step 5: Look at the radio waves region.
Radio waves have very long wavelengths and carry almost no useful detail about the surface, so this region is not used in normal remote sensing.
Step 6: State the answer.
The region that cannot be used is the radio waves region.
\[ \boxed{\text{Radio waves region}} \]