Temperature and silica content determine the of magma.

I think the correct answer is that they can determine the viscosity of the magma. The viscosity of a magma is largely controlled by the temperature, composition and the gas content. Also, silica content can define a magma type. It is said that higher silica content magma has a higher viscosity than those with lower silica content.

By dividing you can find the answer

The answer would be a