Which wave travels slower than P waves in solid rocks?

Correct answer: S wave

In solid rocks, S-waves travel more slowly than P-waves. P-waves are primary, compressional waves and usually arrive first at a seismic station, whereas S-waves are secondary, shear waves and arrive later. The question compares the two main body-wave types in seismology. Radio and light waves are electromagnetic, and the general term sound wave does not identify the relevant seismic wave in this comparison, so S-wave is the unambiguous answer.