ANS:D - All the above.

1. Reducing the space occupied by air within the soil: When soil is compacted, the spaces between soil particles are reduced, which decreases the amount of air within the soil mass. Compaction techniques like tamping, rolling, or vibrating soil layers can help achieve this. Less air space means higher soil density.
2. Elastic compression of soil grains: Soil grains can be compressed closer together under pressure. This compression is elastic, meaning that the grains will return to their original position once the pressure is released. When soil grains are compressed closer together, the soil becomes denser.
3. Expelling water from pores within the soil: Water occupies pore spaces between soil particles. By removing water from these pores, either through drainage or evaporation, the soil mass becomes denser as the voids previously occupied by water are eliminated. Techniques such as compaction or applying pressure help in expelling water from the soil pores.