ANS:D - All the above. .

1. The dry density reduces by addition of water after attaining optimum moisture content: This statement is generally incorrect. During compaction of soils, the dry density typically increases with the addition of water up to a certain point called the optimum moisture content. Beyond this point, further addition of water reduces the dry density because excessive water can lead to saturation of soil particles, increasing the void ratio and reducing the density.
2. The line joining the peak of three moisture content graphs obtained by using three compactive energies is called line of optimus: This statement contains a terminology error. The line joining the peak of three moisture content graphs obtained by using different compactive energies is typically referred to as the "line of optimum," not "line of optimus." The line of optimum represents the optimum moisture content for achieving maximum dry density during compaction.
3. Well-graded coarse-grained soils can be compacted to a very high density as compared to fine-grained soils: This statement is generally true. Well-graded coarse-grained soils, which contain a wide range of particle sizes, tend to achieve higher densities during compaction compared to fine-grained soils. The presence of coarse particles facilitates better interlocking and rearrangement of particles during compaction, resulting in higher dry densities.