Final answer:
Viscosity does not affect the temperature at which magma forms; it affects the flow and cooling rate of magma.
Step-by-step explanation:
The temperature at which magma forms is affected by pressure, the composition of the source material, water, and viscosity. However, the correct answer to the question is D. viscosity. Viscosity is a measure of a substance's resistance to flow. It impacts how easily magma can move and affects the rate at which it cools and solidifies. The other factors listed - pressure, composition of source material, and water - all play a role in determining the temperature at which magma forms.