The correct answer is - a) viscosity, gas content, and silica content.
The classification of the magma depends on its viscosity, gas content, and silica content. This are the three components that define the properties of the magma, as well as its behavior, and the types of rocks that will form from it after it cools off when it comes out of the volcano. The eruption of the volcano is also heavily dependent on the composition of the magma, so the different types of eruptions that can be witnessed at the volcanoes, are all defined by the magma composition.