Question

some plutons become more felsic at the top. this is because
fractionation
assimilation
partial melting
accretion

Explanation:

Fractionation refers to the separation of minerals in a magma body based on their different crystallization temperatures. Felsic minerals (rich in silica) have lower crystallization temperatures. As a pluton cools, mafic (lower - silica) minerals crystallize first and settle (if they are denser), leaving the remaining magma more felsic. This process can lead to a more felsic composition at the top of a pluton. Assimilation involves the incorporation of surrounding rocks into the magma, which would change the magma composition but not specifically create a felsic - at - the - top pattern in the way described. Partial melting is the process of melting a rock to form magma, not related to the internal differentiation within a pluton. Accretion is the process of adding material (like in tectonic settings where terranes are added to a continent), not relevant to the internal composition of a pluton.

Answer:

Fractionation