Why is the outer core the only layer in earth that is molten? the temperature is high enough but pressure low enough to melt iron. the pressure decreases within earth, thus allowing iron to melt. the temperature is high enough but pressure low enough to melt olivine. the pressure increases within earth, thus allowing iron to melt?

The appropriate answer is a. the temperature is high enough but pressure low enough to melt iron. The earth's core is composed of two layers; the inner and outer core. Both layers are composed mainly of iron and nickel. The main difference between the two layers is that the inner core is solid and the outer core is liquid. The inner core is solid because of the pressure created by the overlaying layers; the outer core, mantle and the crust. The inner core is however hotter than the outer core. 

Dall of the above all of the choices make sense