Correct Answer:
A. Solid
The correct answer is Solid. Seismic S-waves (Secondary or Shear waves) are transverse waves that shake the ground perpendicular to their direction of travel. They can only propagate through solid materials and are unable to pass through liquids or gases.
Properties of S-Waves
- Shear Wave Mechanism: S-waves move by shearing or displacing material at right angles to the wave path. This requires the medium to have rigidity — a property only solids possess. Liquids and gases lack shear strength and will not transmit these waves.
- Evidence of Earth's Interior: The fact that S-waves cannot pass through the Earth's outer core (which is liquid iron and nickel) provides strong evidence that the outer core is molten. Seismologists observe an "S-wave shadow zone" on the side of the Earth opposite an earthquake.
- Comparison with P-Waves: Primary (P) waves are compressional and can travel through solids, liquids, and gases. S-waves are slower and arrive after P-waves on a seismogram.
- Why Not Other Options?: S-waves cannot travel through gas, liquid, or plasma because these states lack the necessary shear modulus. Only solids support shear stress.
Thus, S-waves in earthquakes can only pass through solid matter.