Final answer:
Terrestrial planets have cores made of iron and nickel because during their molten phase, gravity caused denser materials to sink and form a core, while lighter materials formed the crust.
Step-by-step explanation:
The cores of terrestrial planets such as Earth, Venus, and Mars are mostly made of heavy metals like iron and nickel because, during the early formation of these planets, they were once molten. During this molten phase, the process known as differentiation occurred, whereby gravity caused the denser materials to sink toward the center of the planet, forming a metallic core, while the lighter silicates floated to the top, forming the crust. This means that the correct answer to why terrestrial planets have metallic cores and silicate crusts is that heavier elements tended to sink to the centers of terrestrial planets when they were molten (option e).