Final answer:
At a divergent plate boundary, oceanic crust is formed. This new crust consists mainly of igneous rocks, specifically mafic rocks such as basalt, as a result of magma upwelling, cooling, and solidifying.
Step-by-step explanation:
Divergent plate boundaries are locations where tectonic plates are moving away from each other. This movement leads to the formation of new oceanic crust as magma from below the Earth's surface rises to fill the gap, cools, and solidifies. At these boundaries, mid-ocean ridges are formed, which are essentially underwater mountains and rift systems. These ridges are composed primarily of igneous rocks that are mafic in composition, typically basaltic.
As the Earth's peridotite mantle undergoes decompression melting, the partial melting results in mafic rocks, either extrusive or intrusive, which comprise the majority of the oceanic crust. Additionally, sediment accumulation can occur over time, leading to marine sediments overlying the igneous rocks including chert and marine clays. These entire sequences are known as ophiolites and can sometimes be exposed in mountainous regions during continental collisions.
Therefore, in response to the initial question, what is formed at a divergent plate boundary is option C, oceanic crust.