How are igneous rocks formed?
a. lava or magma cools and then crystallizes.
b. glaciers melt.
c. lava or magma heats up and crystallizes.
d. hail falls and melts.

Igneous rocks are formed through the lava and magma cools then crystallizes.
Igneous rocks can also be form without going through crystallization. The occurrence of igneous rocks may undergo with 2 different process; the intrusive or plutonic, or the extrusive or volcanic. Intrusive igneous rocks are from magma that cools and became solid inside the crust of the earth surrounded with country rocks. While the extrusive igneous rocks are those rocks that formed in the crust surface as a result of melting of rocks inside the mantle.