Final answer:
Felsic magma is the most viscous due to its high silica content, which makes it flow less easily than other types of magma.
Step-by-step explanation:
The magma composition that is the most viscous is 4. felsic. Viscosity in magmas is primarily controlled by the silica content; higher silica content results in higher viscosity. Therefore, felsic magmas, which contain a large amount of silica, are highly viscous. These magmas are associated with explosive volcanic eruptions, as they obstruct the escape of volcanic gases.
Mafic magmas, like those that form basalt, are characterized by lower silica content and, consequently, lower viscosity. This allows for a more fluent flow of mafic lavas compared with felsic lavas. Intermediate magmas have a viscosity that falls between the mafic and felsic magmas, and ultramafic magmas, although they are characterized by even lower silica content than mafic magmas, are typically found at very high temperatures, which also affects their viscosity characteristics.
The mineral content and color of the rocks provide insights into the composition of the magma from which they solidified. Darker colored minerals such as olivine and pyroxene indicate mafic or ultramafic compositions, while lighter colored minerals like quartz and potassium feldspar suggest intermediate to felsic rocks.