Question

which element is in abundance in the crust but not in the mantle?
oxygen
carbon
magnesium
silicon

Explanation:

Oxygen is abundant in the Earth's crust. Carbon is not in high - abundance in either the crust or mantle compared to major rock - forming elements. Magnesium is abundant in the mantle. Silicon is abundant in both the crust and mantle. Oxygen is the most abundant element in the Earth's crust and is relatively less abundant in the mantle compared to other elements like magnesium and iron in the mantle's composition.

Answer:

oxygen