What shape should the air dried soil be mixed into after achieving the plastic state?

The correct option is to shape the air-dried soil into a ball after achieving the plastic state. When soil reaches its plastic limits, it can be readily molded and formed into different shapes. A ball shape is particularly appropriate because it allows for uniform compaction and consistent moisture distribution, which is essential in various testing or construction processes involving soil.

Forming the soil into a ball facilitates better handling and storage, as it minimizes surface area exposure, reducing evaporation. Additionally, a ball shape can help in evaluating the consistency and workability of the soil, providing a more accurate representation of its properties when conducting further tests, such as the Atterberg limits or compaction tests.

The other shapes like a cube, cylinder, and pyramid do not offer the same practical advantages in terms of handling and assessing the soil's characteristics when in a plastic state. For example, a cube or a cylinder might lead to uneven drying or make it more difficult to evaluate the soil's integrity when testing for compaction. A ball shape simplifies these processes, making it the most effective choice.