Structures that form from the cooling and hardening of magma beneath earth’s surface are
a. vents.
b. calderas.
c. lahars.
d. plutons.

Plutons. It is 100% correct.

here is the definition of a pluton to prove my point:

In geology, a pluton is a body of intrusive igneous rock, called a plutonic rock.  It is crystallized from magma slowly cooling below the surface of the Earth.

a Plutons.

Explanation:

scientifically proven. dont trust? look at the information.