The correct answer is c) radio waves.
Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used to determine the structure of molecules. It operates by placing a sample in a strong magnetic field and exposing it to radio frequency radiation. When the sample is exposed to this radiation, certain nuclei resonate at specific frequencies which can be detected and analyzed. This is different from techniques that use infrared or ultraviolet light, which are employed in other forms of spectroscopy but not in NMR. Therefore, radio waves are the key to eliciting the resonance that allows for molecular structure determination in NMR spectroscopy.