Answer:
a) earthquake waves
Step-by-step explanation:
Evidence from earthquake waves proves that the earth's outer core is liquid.
We have two broad types of earthquake waves:
Body waves
Surface waves
The body waves travels within the earth and are of two types:
P-waves
S-waves
The surface waves accompanies an earthquake and it moves on the surface. The two types of surface waves are:
Love waves
Rayleigh waves
We use body waves in seismic investigation of the earth due to their unique properties.
P-waves are primary waves that can move through any medium. S-waves or shear waves cannot propagate through liquids and gases as those material cannot be sheared.
If the outer outer core is solid, both P and S-waves would easily pass through. From studies, it has been shown that S-waves are cut off in the region of the outer core and they reappear in the inner core. P-waves pass through both outer and inner core. This proves that the composition of the outer core is in a molten/liquid phase.