ANS:D - all the above.

1. Dry density: Dry density is the mass of solids per unit volume of soil when all voids are filled with air. It is a key parameter used in geotechnical engineering to assess the compactness or density of soil. Dry density is typically measured in the field using techniques such as sand cone method or nuclear density gauge.
2. Zero air voids: Zero air voids refer to a state where there are no voids or air pockets within the soil mass. Achieving zero air voids is desirable during compaction processes as it indicates that the soil is fully compacted and there are no pockets of air remaining within the soil mass.
3. Saturation dry density: Saturation dry density is the maximum possible value of dry density that a soil can achieve when it is fully compacted under ideal conditions. It represents the highest density that the soil can attain and is typically measured when the soil is compacted at its maximum effort and all voids are filled with soil particles.