Final answer:
The increased presence of silica in magma significantly contributes to the increase in its viscosity. Silica creates a complex network that impedes the flowability of the molten material, making it more viscous.
Step-by-step explanation:
The more silica in a magma, the more its viscosity increases. Magma viscosity is affected by several factors, including temperature, composition, and volatile content. However, when it comes to the composition, an increased amount of silica (SiO2) leads to an increase in viscosity. This is because silica forms a complex three-dimensional network of SiO4 tetrahedra in magma, which hinders the flow of the material. As for the provided options, the presence of ferromagnesian minerals typically reduces viscosity, while the introduction of water can decrease the viscosity due to the disruption of the silica network. Air bubbles generally do not affect the chemical viscosity of the magma but can change the bulk or apparent viscosity. In summary, the main answer is silica, which results in a higher viscosity due to the creation of a more complex silicate structure within the magma.