The correct answer is: A. clay
Clay is the most suitable material for the central impervious core of a zoned embankment type
dam because it has the highest impermeability and strength. It is also relatively easy to work with and can be compacted to a high density.Coarse sand is not as suitable as clay because it is less impermeable and has lower strength. Silty clay is also not as suitable as clay because it is more permeable and has lower strength. Clay mixed with fine sand is not as suitable as clay because it is less impermeable and has lower strength.
The central impervious core of a zoned embankment type dam is the most important part of the dam. It is responsible for preventing water from seeping through the dam and causing it to fail. The core must be made of a material that is impermeable, strong, and easy to work with. Clay is the best material for this purpose.