What is one of the reasons why scientists believe the outer core is liquid?

a. p-wave shadow zone.

b. s-wave shadow zone.

c. outer core fish fossils that have made their way to the surface.

d. negative gravity anomalies?

Scientists have obtained most of this information from natural geological activity.
S-waves and p-waves are seismic waves- the waves of energy that travel through the Earth as a result of an earthquake .Both P- and S-waves travel easily through solids, but S-waves cannot pass through liquids, and they cannot pass through the outer core. Scientists know that Earth’s outer core is liquid because s-waves are not able to pass through it. When an earthquake occurs there is a “shadow zone” on the opposite side of the earth where no s-waves arrive. 
So, one of the reasons why scientists believe the outer core is liquid is the s-wave shadow zone.