38. when erosion wears away the outside of a volcano, sometimes a solid magma core called a is left exposed.
a. cinder cone
b. volcanic neck

volcanic neck

Explanation:

volcanic neck

the volcanic neck which is also known as the volcanic plug is a type of land form of the volcano which formed when lava from volcano hardened inside the vent of an active volcano. Due to hardened magma intense pressure is build up which sometimes become the cause of explosion in volcano