Final answer:
Shield volcanoes form from magma with low viscosity and mafic composition, leading to easy-flowing lava that creates a gently sloping profile.
Step-by-step explanation:
The formation of a shield volcano is directly linked to the properties of the source magma. Specifically, a shield volcano typically forms from a magma that has a low viscosity and is of mafic composition, which means it is rich in magnesium and iron. The low viscosity allows the lava to flow easily across long distances, which contributes to the gently sloping profile of shield volcanoes. Mafic magma also tends to have a higher temperature and fluidity compared to felsic magma, aiding in the spread of lava. The correct answer to the question is therefore: c) low viscosity and mafic composition.
Other compositions, such as those with high dissolved volatiles, high viscosity, or an ultrafelsic nature, do not typically lead to the formation of shield volcanoes. Felsic magma, for instance, which is mentioned in the question about rhyolite, has a high viscosity and consequently forms more explosive volcanic structures, such as stratovolcanoes or lava domes. Due to this high viscosity, felsic lava does not flow as easily and thus does not form the characteristic shape of a shield volcano.