Final answer:
Limestone has the largest capacity to naturally remove sewage pollutants due to its ability to filter water, neutralize acids, and provide spaces for biological activities. Other rock types like shale, sandstone, and granite are less effective due to their varying levels of permeability and density.
Step-by-step explanation:
The rock type that would have the largest capacity to naturally remove sewage pollutants is d) Limestone. Limestone is known for its ability to filter water, primarily due to its chemical composition which allows it to neutralize acids and balance pH levels. This property can help in removing contaminants from sewage by precipitation and the addition of alkalinity to the water. The porous nature of certain types of limestone, such as chalk, travertine, and oolitic limestone, enhances its ability to filter and purify water by providing space for biological activities that naturally break down organic matter and pollutants.
Other rock types mentioned, such as shale, sandstone, and granite, have different properties. Shale has a very fine grain that makes it less permeable, hence providing minimal filtration. Sandstone, depending on its type, might have some filtration capacity due to its porosity, but generally not as much as limestone. Granite, being an igneous rock, is very dense and does not have a significant capacity to filter water naturally.
Filtering pollutants from sewage can also involve biological and mechanical processes such as sand filtration and the action of bacteria, algae, and zooplankton, which are part of natural and engineered wastewater treatment systems. However, when considering the natural capacity of rock types to remove pollutants from sewage, limestone stands as the most effective among the options provided.