Final answer:
Secondary waves, or shear waves (S-waves), cannot travel through liquids such as the liquid outer core of the Earth. They can only propagate through solids, meaning the correct answer is b. liquids.
Step-by-step explanation:
Secondary waves, also known as S-waves or shear waves, cannot travel through liquids. This property of S-waves provides essential information about the Earth's interior structure. Since S-waves are transverse waves, they require a medium that can support shear stress. Earth's core, which contains a liquid outer layer, does not allow transverse waves to propagate through it. However, S-waves can travel through solid materials like rock, Earth's mantle, and Earth's crust.
Given the options provided, S-waves cannot travel through option b. liquids.