Magma forms when solid rock in the crust and upper mantle
a. partially melts.
b. crystallizes.
c. vaporizes.
d. cools.

The correct answer is letter A: Partially Melts. Magma is formed from partial melting of existing rock, coming from two sources: in the upper mantle or the crust. 

Explanation:

We call partial melting to what happens when only some parts of a rock melt. this happens because rocks are not made up of pure materials.  Several rocks are composed of many materials, and each of them has a different melting temperature.   The opposite of partial melting is complete melting, for example, the ice under the sun, or the wax before the fire. There are two ways in which rocks melt: Decompression melting and flux melting. The first one happens within the earth when a rock is held at approximately the same temperature but the surface is reduced. 

Flux melting appears if a rock is close to its melting point and come water is added to the rock, then the melting temperature is reduced and partial melting starts. 

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