Scientists infer that earth's outer core is liquid based on their studies of

a) earthquake waves
b) magnetic polarity reversals
c) lithospheric heat flow-
d) magma

a) earthquake waves

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.