Final answer:
Color does not affect the viscosity of magma, whereas temperature, composition, and pressure are the primary factors that influence it. Color is only a reflection of the minerals in the magma and does not impact flow resistance. Option D is correct.
Step-by-step explanation:
The factor that does not affect the viscosity of magma is D) Color. Magma's viscosity, which is a measure of its resistance to flow, is primarily influenced by three key factors: temperature, composition, and pressure. Temperature plays a significant role as hotter magma tends to be less viscous. Composition affects viscosity through the varying amounts of silica present, with higher silica content leading to higher viscosity.
Pressure influences the melting point and the behavior of the magma; as pressure decreases, magma can become less viscous since it can start to expand and form bubbles. However, the color of the magma has no effect on its viscosity; the color is more indicative of the minerals present and does not impact the physical properties that control how magma flows.