How are young volcanoes evidence of cracks in
the earth's surface to magma below?

These separating borders create fractures and fissures in the rocks. It is these cracks in the land that allow hot magma to surge upwards and fill the gaps.

the answer to your question is b

igneous rocks which formed by the crystallization of magma at the surface of the earth called extrusive rocks. they are characterized by fine-grained textures because the rapid cooling at or near the surface did not provide enough time for large crystals to grow.